This is a pretty slapdash guide made by someone with 1800+ hours in the game with most of them spent goofing around with ideas with the game just sitting open.

If you just wanna pick what sounds right for you, you shouldn't need to read too far in here, though if you pick hellknight you should know I have an entire separate guide for that with cleaner presentation than the mini-walkthroughs (the one with "Autowin" in its name, not the outdated "hands-off" one).

-I included some general guidelines about making builds and how to deal decent damage etc here
-If you want more details, best I've got is the mini-walkthroughs which aren't entirely mini but they are describing entire runs and I'd argue they're still brief compared to how much was actually done (I didn't exclude any relevant build decisions, just most of the grind & suffering details)
-If you wanna know why I avoided most spells & enchantments a lot, it's because I didn't want anyone to think that winning their run would be dependant on spell or related loot RNG
-^ For the same reason, I used NO artifact traits at all
-^ For similar reasons, I tried & mostly failed to avoid boosting artifacts a bunch; nothin past 30 tho
-I listed a small amount of details on advanced specializations in this guide, too
-There are some pretty great resources on the Discord including a list of errata & bugs & formulas

TL;DR
-You can just skim if you wanna mostly figure strats out yourself
-I suffered extra so that the mini-walkthroughs weren't RNG-dependent (starter spells, no 3rd traits)
-Other helpful details are noted in this guide
-Join the Discord

Animator - Animator focuses on empowering just its Animatus (starter creature for Animator) and the rest are merely support. You will primarily be boosting its stats as much as possible to let it hammer enemies, and among the most common methods are to make it start out as large as possible and snipe out just one enemy at a time to start a chain reaction that wipes the rest, or sacrificing ally creatures to set the chain off.

Bloodmage - You're here to stack MASSIVE amounts of maximum HP and that's the ONLY stat you need for EVERYTHING. It has some missing HP perks but those are just a bonus. Turn your brain off and stack more HP. If that's not enough stack even more. One of the most broken specs. (TIP: Titan's Wound has a lot of REALLY great creatures with high base HP and some other benefits that are great for making beefcakes)

Cabalist - Random bs GO. Seriously just make your team stack as many ethereal spell gems as possible for apex and shimmer (2 of its perks) to put in work to boost your damage and tank power and cast all the random nonsense to scale up while debuffing enemies. Ironically in terms of damage, sourced indirect is your best option (will explain what this means later; Djinn Pyromancer is a strong example though).

Cleric - Reduce damage, increase healing, spam healing-based effects as much as possible. There will be a lot of triggers going on but just stay alive and keep healing and scaling and you should probably win. Also barriers & the additional damage based on them is GREAT. (TIP: The spirit race is your best friend and shows up early)

Defiler - Enemies are just flat out much weaker to start thanks to stat-reducing perks. Load them down with random debuff spam and stat reductions and any perks that synergize with it and you should be fine.

Evoker - The foxdoggy race, "vulpes", is literally made for you. Abuse huge base intelligence values, spells with big potency values, and later on, spell enchantments. Preferably you'll want to use magnetic specifically, but realistically during the story you won't be able to afford much. So if you can enchant magnetic onto just one spell, I recommend you save it for a super high-potency finisher spell to nuke bosses or just really pesky tanks with.

Hell Knight - Attack. Yep that's basically the whole thing. Use creatures with BIG base attack values to hit enemies hard with your attacks, hold on to a few attack spells (molten armor is a starter spell and is perfect for when you need scaling on your attack), and maybe a bit of tank power to hopefully avoid getting bursted down (bolster keeps you alive as long as you don't get bursted to death).

Inquisitor - Heal the enemies to death, and heal your own creatures for powerful scaling. Censure is VERY good. Problem: you can't heal while at full health and unlike cleric you don't have overheal to solve that. So you'll wanna do something like cast a spell to increase your max HP and THEN you can heal. Once you hit depth 30, Koloss Doombringer will enable you to basically ALWAYS trigger healing once you've done it just once that battle. So that's a crucial unlock.

Monk - Crosswinds. You literally ONLY need speed for damage. That perk is THAT busted. If you're not dealing enough then you need more speed or bigger hit counts (AoE is best). Also you dodge and stuff I guess... (TIP: Hunter race is good, and yes crosswinds adds to their trait damage)

Necromancer - Have all the different random minions you can get to pull off one of the ONLY sources of 100% damage reduction in the game (most cap at 90% even if they don't tell you). No particular plan aside from that, really. Your minion damage is based on your creatures' stats though, so you should maybe bring scaling (and maybe focus on a favorite minion for said scaling). (TIP: Necro Grimoire unlocks at depth 30, and it's really great for getting the minion count up fast)

Pyromancer - Scales fast as hell as long as you don't die and enemies aren't immune/don't keep removing burning & thus causing its potency to reset. Once you reach Vulcanar's realm (Great Pandemonium) you'll wanna farm favor to unlock GOD-TIER spells for this nonsense. Raze (deal 2x enemies' burning potencies to them in spell damage form) & Inferno (Increase enemy burn values by 100%). Exponential scaling. Absolutely busted.

Reaver - Fatigue is your main source of damage. It's a really slow one. Its starter is ironically redundant once you have most of your perks, so don't fall into the trap of thinking the apocalypse creatures are actually good with it in general (the debuff resisting one and the %less damage one are fine, at least). Make your build NEVER die and you'll basically always win (be sure to abuse the fatigue-based perks first for your damage scaling).

Sorcerer - Not recommended. No, it's not a "caster" like you would expect from its name and presentation. It's basically built to be a stunlock specialization, but it doesn't do the best job of that when it counts, particularly in the late game. Anyway when you see things that give snared or other debuffs, take them. Your perks won't really help you scale your stats so you're gonna have to figure that part out yourself using spells and traits for it (shouldn't be hard, there's a lot). Your biggest power spike will be the very start of the game when you can just get free stun debuffs (less effective later), and depth 60 when you unlock Bard Trovatore (for certain reasons this nearly guarantees that you go first even against "Agile" nemesis enemy groups and is 100% first move against enemy mimics; windrunner can also do this but it's an advanced spec and normally uses speed for that anyway).

Warden - Spam all the buffs all the time. Also get randomly countered sometimes. Honestly there's not much to say, it's pretty strong just by using its perks to the fullest. Its only real weakness is enemies that PREVENT buffs, and there's not a whole lot you can do about that aside from having your team be flexible enough to handle that situation (which could mean using any number of things, but normally if you can't out-scale enemies with self-buffs you can at least debuff and weaken enemies, so...).

Witch Doctor - Why are you hitting yourself? This one is kinda weird because it requires particular types of spells to be effective, like Mind Control. This means you'll start out in a weird spot and slowly reach a point where you can actually use its perks. Try to just focus on the one that casts mind control on-attack at first, and then the one that makes enemies hurt each other using their highest stat. You also don't want enemy damage stats to be weak since you want them to hurt themselves, so when you reduce their stats you want to JUST lower defense. Your best tools will come from random spell unlocks and a post-story trait called "Sarea's Command".

Number one rule: stats are king. Most of your problems can be solved by bringing even bigger stats to the battle, and for casters bigger spells as well (still stats tho). If you're struggling and can't see any way to improve, then grind for more levels (and maybe new spells) and come back bigger and hitting harder.

PROJECTS:
First and foremost any time "maximum projects" is available you take it IMMEDIATELY. And if not you upgrade the blacksmith. If you don't have at least a maximum of 3 project slots or some extra time on your hands don't even bother taking other projects. The next most important one is the enchanter for casting specs, but otherwise it's just unlocking other various tools for your castle (e.g. the gambling dwarves have a shop with nice stuff in it).

Making a BUILD:
-Most builds only need 1 to 3 creatures to carry their damage, while the rest play support. This means half your team generally won't even need artifacts, so you can usually just give them helmets to try to keep them alive longer if you feel inclined to boost them.
-Make sure you have a reliable plan to help your carry actually hit stuff before it dies. This may involve using hotspur or until the end (salamander traits) to cast damage spells or to cast something like 'Panic Attack' if you're an attack build, or just tanking hard enough to get turns the normal way, or stacking speed to go first.
-Probably an obvious one, but look out for things that synergize with your perks or with other traits you've seen before
-If you die and don't know what happened, you can check the battle history to hopefully figure out what killed you and look for a way to counter it assuming it wasn't just really bad luck that you can ignore (if you win 99.9% of fights then you can just not worry about that last 0.1%, you have a good build)
-NOTE: You will unlock a new creature for every race, every 15 depth that you climb. Most races will be filled out by depth 90 or so. All of them by 150.
-ALSO NOTE: You don't necessarily want to replace creatures too much from depth 30 to 42, because leveling will be slow until you unlock the LEVEL UP FEATURE at the fusion lab at depth 43; thus if your build is struggling, this is probably the last time you should consider swapping things until then.

PERSONALITIES & SUMMONING NEW CREATURES:
-Personalities affect how your stats scale up per-level on a given creature
-Stats normally gain +30% of their level 1 value per level
-The boosted stat is +40% of its level 1 value per level
-The reduced stat is +20% of its level 1 value per level
-It's completely random
-When Summoning a new creature, you need to have killed that creature in a normal encounter or by interacting with the object that starts a fight against a group of them
-You can have 6 of the same creature, it's fine
-After summoning a creature, you will have 0% mana for them and thus need to enter a realm to kill another one
-If you don't like the personality on a creature, note that you can manually close the game without saving and start up again instead of entering another realm to try summoning it again

FUSION:
-The game explains this to you outright, but I'm noting it here because it's related to the above
-Main parent (first creature you select when fusing) gives the race & sprite AND PERSONALITY
-Second parent (2nd creature you select when fusing) gives the class & color palette
-BOTH creatures' traits are retained. That means it's ALWAYS strictly better (as long as you didn't ruin the base value of their main stat somehow)
-Stats are AVERAGED based on their level 1 values, experience is averaged (experience and levels scale a bit weird, try not to think about it)
-It applies retroactively, so no the timing of fusion doesn't mean ANYTHING WHATSOEVER

How to SPELLS:
-You start with a set of a few specific spells, 3 for each class, and it's always the same ones not random or anything.
-You find spells randomly from loot in realms, with some realms having objects which are much more likely to yield them (including a blue shrine stone lookin thing that can appear in all realms)
-You also find spells in the shops that gods have in realms, and they unlock more based on your favor level with that god. To see a god's shop you have to interact with their statue in their realm.
-Upgrading spells does nothing on its own aside from unlocking enchantment slots, and in the case of SPECIFIC enchantments it boosts the % value for those enchantments.
-As explained in the codex, 'small', 'moderate', 'large', 'massive', & 'devastating' will use different amounts of the caster's INTELLIGENCE to apply the effect. For example, damage spells, which also have innate defense penetration, have these values: 90% INT + 0% pen, 110% INT + 10% pen, 120% INT + 20% pen, 130% INT + 30% pen, and 140% INT + 40% penetration respectively.
-IMPORTANT NOTE: Spells which affect an entire team without anything needing to be targeted are AoE spells, which are one FOURTH as effective as targeted spells (normally referred to as single-target spells, but this might be confusing when you see a spell like cloud beam which is treated as an AoE despite only hitting one target at a time, vs a spell like chain lightning which can also hit every enemy but is considered single-target). For damage spells, this doesn't mean AoE struggles more against enemy defense, it means the damage dealt AFTER defense is accounted for is reduced to 25% of the damage it would otherwise deal.

BOSSES: Target down the boss as hard and as quickly as you can (unless it's Giran, then you do the opposite). If that doesn't work bring bigger stuff and do it again. Or just try again and hope whatever happened the last time was just a fluke, or that maybe you can luck into a victory this time. You can also try to counter their mechanic somehow, such as casting Dispel/Mass Dispel/Greater Dispel against QILA who is notorious for being really annoying to kill otherwise.
TIP for Ramses (depth 48): Apply blighted to him somehow (affliction would eventually work). Dies on his own like that.

The in-game codex, accessible from the menu, explains a lot of things, so consider looking at things in there as a lot of important information is in there.
NOTE: Difficulty is at 1 by default, and there is no benefit of any kind for fighting at a difficulty higher than 0, not even more experience from fighting higher level enemies. So you can safely set this to 0 if you want to clear the story faster. The most significant challenges are barely even affected by this.
Minor note: Getting favor with gods to unlock items to buy in their shops will unlock the trait material(s) for the corresponding creature(s) in their shop a few ranks AFTER you unlock the actual creature itself (usually about 3 ranks after).
Consider joining the DISCORD server if you need specific details on something.

DAMAGE (something that gets really funky to actually calculate):
---Attack damage is attacker's attack stat minus the target's defense stat, but with a minimum of 1% of the attacker's attack value dealt as damage BEFORE things that modify the damage by a % value are applied. So 1000 attack vs 1000 defense with 50% damage reduction would be 1% of 1000 attack = 10 damage, then reduced by 50% to 5 damage.
---Spell damage is some % of the caster's INT (intelligence), with an innate defense penetration value, based on the spell's stated potency, unless the spell states otherwise. Arrow spells, for example, use speed instead and tell you so. The values of the potencies are mentioned in the codex & in a previous section in this guide. Potency enchantments multiply the amount of INT used for a spell when applicable. However, other effects that mention boosting "potency" do not, they instead multiply the value AFTER it's been calculated based on INT (and for damage it's also after defense is subtracted, making it inferior to enchantment-based 'potency' multipliers). As the codex explains, magnetic is a 15% boost to (base) potency based on the number of copies of that spell on the team, but what might not be clear is that it counts ethereals (I don't believe this was always the case, and might not be intended, but late game it's kinda crucial to making INT-based casting actually viable in comparison to something like bloodmage HP stacking).
---Indirect damage is literally EVERYTHING that isn't an attack or spell specifically. This includes damage that is triggered FROM an attack and dealt via a trait such as Crypt Bat's "Bombardment". There's a difference between "sourced" indirect and "sourceless" indirect. Debuffs (and some traits) are "sourceless" indirect, and thus things that make your creatures deal X% more damage don't affect them (they also don't trigger damage reflection effects). Things like Crypt Bat's "bombardment" and Inquisitor's "Flagellation" perk are sourced indirect damage and thus benefit from things that make your creatures deal X% more damage. A general rule is if an effect says "this creatures deals X damage" on a trait, it's probably indirect damage sourced to that creature. To be clear, something like Valkyrie Scout is just an attack which deals lower post-defense damage, it isn't indirect. Triggered actions and AoE spells aren't indirect damage (this gets asked about on Discord from time to time).

NOTE ABOUT % boosts in GENERAL:
-If confused about something that happened in-battle, consult the battle history (Hit the third button, which is F on keyboards, to copy it to your clipboard if you need it; screenshots if you can't)
-If you hit a creature with barrier and don't break it, then even if you damaged the barrier the game will NOT list the damage as being greater than 0 in the battle history
-%damage & other increases ADD with most other such increases
-%damage reduction & other decreases MULTIPLY (100% damage vs 50% less & 50% less = 75% less damage dealt for a remaining 25% of damage dealt)
-%damage modifiers happen AFTER defense is subtracted for attacks and spells, so if your stats are lower than enemy defense you'll deal hardly any damage even with 1000% more damage dealt (thus bigger stats matter a lot more)
-%damage modifiers happen BEFORE defense is subtracted for indirect damage, which means that indirect damage benefits WAY more from these multipliers than attack or spell damage. It also ONLY subtracts 20% of enemy defense from the damage. Sourced Indirect damage can be very powerful for this reason.
-DIMINISHING RETURNS apply to stat gains after reaching 500% of the base value in that stat. Artifacts do not affect the base, so it's as if you gained those stats in battle, thus bringing you closer to that 500% (but they don't trigger on-stat effects and can be stopped by things like the feared debuff). You shouldn't worry too much about this limit, as the values don't just drop off a cliff; they go down slowly, hence 'diminishing' returns.

TRIGGERS (something not mentioned much in-game):
"On-attack" refers to things that trigger after a creature attacks
^ In order for an on-attack effect to trigger, like unicorn vivifier, the attack MUST damage HP
^ This means NO dodge, NO barrier or shelled buff, & damage dealt MUST be at least 1
^ "When-attacked" is functionally identical, but is a reaction to being attacked instead
"On-cast" refers to things that trigger after a creature casts a spell
^ This only requires that a spell be cast to trigger, and is thus far less limiting
^ "MANUAL" casts mean specifically when you select the spell to cast from the menu yourself
^ Anything that doesn't specify "manual" cast can trigger off ALL separate cast instances (gets weird)
"On-defend" refers to things that happen after a creature defends
^ You can't defend while already defending (you can still provoke)
^ You CAN be both provoking AND defending at the same time
"On-provoke" refers to things that happen after a creature provokes
^ You can't provoke while already provoking (you can still defend)
"On-turn" refers to things that happen as the result of a creature starting its turn
"On-heal" refers to healing a creature that is BELOW full HP; overheal from Cleric gets around the full-HP requirement (and paradox from Dreamshade does, which is an 'advanced' spec)
^ You get the full value of the on-heal no matter how much HP was actually restored
"On-stat" refers to things that trigger after a creature gains a stat

ACTION LIMITS:
EACH creature can do a number of different things 15 times EACH turn, with the exception being resurrections where you can only resurrect EACH creature up to 10 times.
In just ONE creature's turn, your team could hypothetically do ALL of these things AT ONCE:
-Attack 15 times each (for 90 attacks from your team total)
-Cast 15 times each (for 90 spells from your team total)
-Gain EACH of the 5 stats 15 times each (for 90 times for EACH stat, for 450 total stat gain instances)
-Heal 15 times each (90 heals total)
-Take damage 15 times each (this means your team casting 90 total AoE damage spells in a turn would end up dealing 0 damage to all enemies at least 75 times)

These are my haphazard explanations of how I handled each starter spec to beat the story with. There are definitely other ways of winning but these ones were how I won. If you want advice on late game builds consider asking around on the Discord.

Evoker:
If your starting creature has an intelligence-down personality... Restart the run. She's a GREAT carry if you don't get 1-in-5 screwed by bad personality. Had to reroll my run TWICE over that, but luckily 3rd was INT-up instead (woohoo). Summoned 2 bards, 1 ent, & 1 vulpes. None INT-down. Equipped magic missile (sorcery) /chastise (life) /poison arrow (nature; so the ent) ASAP. Set all strategies to cast (Menu > Creatures > Strategy). Fought Loid at level 10 with outspoken & spellpower perks. Won, fused bard onto INT-up Vulpes. Got Echo & maxed spell power at Magi's realm, fused phase knight onto ent, equipped 3rd damage spell on whole team. Summoned the two ravens & fused them, summoned fire salamander & fused them onto my bard & replaced unicorn with the sorcery beacon fused with djinn pyromancer. Bursted Qila down, was really funny. Eventually got chaos mastery and the arcane fortitude & arcane power perks, mostly for more tank and the chaos spells I had seemed like the best at the time. Then efficiency. Grinded to level 84 and fused carry onto Avaritia sin with +INT personality put gem slots and spell potency on artifact trick slots. Proceeded to trounce final boss.
NOTE: Gorgons are good sorcery creatures to use as 2nd parents if you want a bit more %reduction (or a bit more %damage). After the story at depth 75 you'll unlock another helpful one that starts enemies off with stone each battle. Efflorescent ent can give a bit more tank if needed, as well. If struggling with the boss, consider these things and recall the level up feature (unlocked on depth 43) at the fusion lab. Unguided are great for tank power as well (life type though), bought from Perdition in Sanctum Umbra (have to grind favor first).

Hell Knight:
Made a whole separate guide on this one already.

Monk:
Stacked crosswinds, then the dodge chance, then celerity, then chance to attack when enemies dodge, Thylacine's Fury, and the other damage stuff; when I reached Ramses I had 400+ spare perk points so I grabbed the indirect prevention chance perk. Summoned ebony ent, grinded and targeted bosses down with rank 10 boots on all 3. Then summoned a terra vulpes and 2 fire salamanders (and equipped damage spells when I could, especially AoE and multi-hit stuff) when I hit depth 8 to help me the rest of the way to swamp area where I got 6 hunters with speed-boosted personalities to fuse with the team. Cleared all the way to depth 45 and then got a faith hunter + bat combo (the one that deals 35% of its attack to an enemy after an ally attacks) and leveled it. That was the team I cleared the story with (Ramses was a bit painful but nothing a bit of leveling and trying again couldn't eventually solve; rushed story in about 2 hours, 10 minutes).

Inquisitor:
Started with just summoning anything with healing or damage reduction, maxed censure first. Then got shining force. Then Defy Evil, Begrudge, Heresy, Divine Strength. Then the rest. Once I reached Titan's wound I got 5 sand giants, 4 with HP boosted personalities. Team was:
Valkyrie scout + SG (Sand Giant), Unicorn + clutcher, SG + ancient spirit, SG + Ebony Ent, SG + Oculum Leech, SG + Blood Crusader (easily replaceable but should be a decent tank). Defending while building up censure (and sometimes using provoke to protect) was an effective strategy on Qila and Vitja (bosses). This build worked for the rest of the story (with occasional suffering, mostly blighted). Used molten armor toward the end just for faster scaling (& shelled is nice too), greater dispel, and a random AoE spell (helped sometimes). FYI you can attack an ally with your unicorn thing to resurrect allies (and to give missing HP for on-heals). Took 3 and a half hours to rush story.

Cleric:
Works similarly to inquisitor but with more focus on barrier stacking, and for speed clears later you have a healing vulpes as an option (unlocks depth 15, just return to blood grove). First perk focus was Apotheosis, Revelation, and Overheal. Then once reaching swamplands IMMEDIATELY HARD PIVOT TO BARRIER AND DAMAGE BASED ON IT. I cannot emphasize enough how painful it was to use a slow build in that place. When you reach Qila, consider the constriction spell on her while you whittle her down (you'll want at least one nature type for this unless you enchant a class swap onto it). Also this was the build:
Ebony Ent + Ancient Spirit, Centaur Ranger + Unicorn Vivifier, Necrotic Clutcher + Unicorn Holycaster (+ATK personality), Necrotic Clutcher + Blood Crusader (+ATK personality), Oculum Leech + Sand Giant (+HP personality), Dusk Ossein (filler). Swords on the clutchers, helmets on the rest.
^ Didn't have decent healing spells for the healing vulpes at depth 15 and didn't wanna grind for them so I just waited until I hit depth 43 and could thus level things up. Or rather that was the plan but I forgot and beat the story without replacing or fusing the ossein...

Warden:
Shellbust and Constriction are gonna be your main spells here. Shellbust doesn't deal huge damage but it does ignore defense, and constriction is an incredibly useful spell for tough battles. Thoughtful targeting with these spells (summon 1 ent & 3 centaurs ASAP) will carry you well enough through the early story. You'll want the ent to be tanky and at least one centaur to have an attack-boosted personality, the other 2 can mostly cast. For perks, you want Invocation (gain arcane after casting) first. Then basically whatever feels right. Don't forget you can reset whenever. Replaced my unicorn with Red Storm after reaching swamplands. Much later, Imp Impington was when I remembered I was supposed to set difficulty to 0, and then I ended up winning just from snared (Constriction spell) + centaur ranger abuse again. Probably wouldn't have even needed to if I had fused my Centaurs ages ago already. FYI the best fusion here is probably Red Storm in the swamplands, just to help you get initiative and start snaring and building up cast counts for the centaur attack volleys. Least important stats for your team are probably defense, HP, & intelligence, in that order (INT is good for shellbust though). Speed to go first and attack just for more damage (in that order) are more important. FYI the perks I was running were primarily geared toward getting buffs as often as possible to trigger the on-buff things, and the perks that gave free casts (constriction and shellbust!) and later on attacks from gaining buffs. Any spec can abuse centaur, but this one is probably one of the better ones at it (also buffs are pretty good in general). With the amount of buffs and trigger spam I was getting by the end of the run, I was mostly defending and letting them explode on their own. Won at about 4 hours without even fusing anything. Hilarious spec when enemies don't prevent you from gaining buffs (that's foreshadowing for late game).

Witch Doctor:
Just summoned whatever at first for full team (centaur and ent not bad though). Fire salamander with secret stuff (Bad medicine perk) goes hard. Fused Centaur with unicorn. Grinded spells in caustic reactor for friendly-fire type spells and got crushing impact which was amazing for boss-sniping paired with the perk that makes enemies use their highest stat for damage with attacks. Reset perks a lot based on need early on (spare points always in trance and/or hedge magic, not really much choice). Got Flood of Darkness in Refuge of the Magi and went nuts with it, absolutely busted spell with witch doctor. It was basically GG from there. Took 2 and a half hours, could've been longer if more spell grind was needed. You can make it work with just your perks & starter spells but... the good spells are worth the grind for this one, 100%.

Won't be doing:
Animator (there's another lad on discord who intends to handle that themselves at some point), bloodmage (it doesn't need one, just stack exclusively HP & abuse 'Voracity' perk), Necromancer (might need its own), Pyromancer (it doesn't need one; once you reach "Great Pandemonium" grind favor ranks until you unlock "Raze" and "Inferno" for purchase in the god shop and abuse them, they're crazy), Sorcerer (I don't wanna, so just spam snare stuff & use Gorgon Gazer; you can unlock other specs at depth 19 btw, with NO cost to you other than time & granite for the project & you don't even have to build perk points back up or ANYTHING).

Cabalist:
Started out summoning 3 centaurs & 1 ent. They carried me to depth 19. Perk order I went with was shimmer, wild magic, apex. I then grinded a bit to get this build up and running - https://berated-bert.github.io/siralim-planner/?b=6cbb4c670bf2_473886ea7ac0_1816a513933a_2b326610e172_a5d1017fff82_9a1d2ad16a1a_&s=CA&r=______
^ Order of importance of stats: INT > SPD > HP > DEF > ATK
Spells were just elemental tome on every ally. Yep, that's it.
Only had 3 artifacts, and one was the starter sword (I never attacked). An INT/SPD staff on the storm + an HP/DEF helmet on the 2nd lich (he kept dying too quickly).
Got stat-checked on Impington, then after entering the 3rd attempt I remembered I hadn't lowered the difficulty to 0 yet. Still won that time... Level 31 vs 45 was tough but I had just enough speed/INT. I also forgot I had perk points so after winning I got silenced immunity (this was much-needed...), 'Mystify', & 'Invoke'. After reaching the boss in Eternity's End, I died many times to my own shieldbearer, and realized the damage shared is before reduction and despite HUGE reduction seemed to still wipe me. This trait is bugged so ♥♥♥♥ that noise. Replaced him with Gorgon Gazer + Phase Knight instead (was hoping I'd just get stone randomly, and somehow I often did). After reaching the beach realm I decided that while I was able to win pretty handily at this point I didn't care enough to waste another hour stress-testing my stats, so I just set the difficulty to 0 already. After reaching 43 I replaced Luna Beacon with a Luna Beacon fused with raptor occultist instead & equipped the other 2 starter sorcery spells the the team just to power its trait up. At 57 I took the Prism, Spellslinger, Temporal Concentration, & Hand of the Magi perks. Destroyed every encounter from here on.
-I could have done this WAY more efficiently if I had been less stubborn about various things and actually paid attention to my perks more. Ended at like 4 and a half hours.
TIP: Use the 'generous' enchantment on your gems to get free bonus ethereal copies of those gems at the start of battle & thus scale much quicker.
Post-Story tip if you wanna stick with cabalist: At depth 75 you'll unlock mitida ophan, which is INCREDIBLE for getting your ethereal count high quickly, and at that point you may want to transition to a life-class build (which may involve just a bit of HP stacking but unlike bloodmage it wouldn't be your main focus). You can also grind favor with Aurum (Temple of Lies) to unlock Whitestar Gemling in his god shop, which is a far-inferior but much more flexible trait for ethereal shares.

Defiler:
Started with summoning 1 ent & 3 centaurs with constriction on all of them again. Carried me until I was able to make this build - https://berated-bert.github.io/siralim-planner/?b=496ec07cab9a_3d4d39d8f3b9_3d4d39d8f3b9_3d4d39d8f3b9_473886d8f3b9_473886d8f3b9_&s=DE&r=______
Literally just had boots on the storms and the starter sword on the wight. Wight had all 3 starter spells, the other 5 had affliction. Focused on Unholy Night (once I had the perk points to afford it, that is), and then curse of lethargy & curse of vulnerability. Then Wane > Curse of Fragility > Daybreaker. Qila required a perk reset to use Impiety and Lingering Sickness instead (after that it was hilariously easy). Personality on wight was +HP, the rest had +DEF personalities (NOT speed-down ones on the storms, though, as they need to go first). Kept spare points in Languish at this point whenever I had some, & the next perks I went for aside from that were Hopelessness > Infirmity > Nighttaker > Tragedy > Tenderness > Anemia > the other two curse perks. Struggled slightly on final boss so I enchanted chance to cast twice on the affliction spells that my storms had (only had 2 of the needed material), and out of annoyance finally gave artifacts to the lizards (helmets). Tried a few more times and almost won, finally gave in and swapped out a lizard for Amphisbaena Chef + Amphisbaena Arbitrator (yes debuffs still last forever even with the snek arbitrator). Slowly beat the devil out of him. Realized I forgot to level up the snek & never lowered the difficulty from 1 to 0... Oh well, that works. The run took about 3 hours.
Post-Story tip if you wanna stick with defiler: Master of abominations is PERFECT for defiler, so try to kill an abomination master (they only show up in one of the realms their race is native to, at your highest depth, and only one per depth so you can't keep re-clearing the same realm even if it's still your highest). Also, raven defiler with your perks does funny math that results in 49.2% current HP on enemies (don't ask how), which Master of Reapers can be fun with (not meta but fun).

Reaver:
This spec has no particular synergy with any traits (don't fall for the trap of thinking the apocalypse creatures synergize with it, they're actually just redundant by the end of the story and thus only maybe the damage reduction one is helpful). So unfortunately your game plan is basically "Don't die (yet)" and the rest will hopefully work out. This is a slow one. I started with a satyr, ent, & 2 centaurs all with constriction again (snared centaur is such a great early game setup), also molten armor on Inox & holy bulwark on unicorn just because. One round of constrictions into centaur attack worked on Loid (molten armor'd it, & +ATK personality). Realized Sand Giant is good sustain & nature type, so once at Titan's Wound I summoned 6 of them to fuse with things. Team was now: Oculum Leech + Sand Giant, Ebony Ent + Sand Giant, Ancient Spirit + Sand Giant, Inox apocalypse & 2 more giants because I couldn't decide. Sword on Inox, helmets on the rest.
^This was a dumb move in hindsight as the time it took to do this I could've had better build options available and constriction centaurs could have probably carried me there but oh well.
Perk order: Onslaught, Reverberation, Controlled Anger, Rigor, Stimulation, Rapid Exhaustion, Red-Eye Fight, Sap, Mental Clarity. After reaching imp impington, had to lower difficulty (forgot to do it earlier yet again). After reaching Eternity's End I started dying EVERY encounter, so I started revisiting previous realms for better traits. Then realized I had grinded to level 39 and thus might fare better (was at level 35 before), tried again, and did fine. Managed to slow-fight my way to depth 45 just fine, somehow beat boss (constriction doesn't work on him, only his allies, but worked out). Might need a build with more tank now, as 43 unlocked leveling & 45 unlocked some good new creatures in realms... So I built - https://berated-bert.github.io/siralim-planner/?b=6d4bd1958e70_47d9b2f571d6_8ff269670bf2_1816a5a32e20_74f06f5bed7c_306029d7cfa0_&s=RE&r=______
Beat final boss 2nd try with this (played it smarter). Took about 6.5 hours to clear story... Phase Knight + Sturdy Gargantuan's good too tho.
TIP: Wisps don't trigger the timeline shift when you have sure-footed. Yes the wording says "enemies", but it really means your creatures can't be forcibly shifted down the timeline. POST-story, consider imlers & imlings, for their %damage reduction available at depth 75, and their mastery traits (beating a rodian master for the first time gives you their trait material; subsequent wins have ~30% chance).

List of advanced specializations:
Astrologer, Brewmaster, Demonologist, Doombringer, Dreamshade, Druid, Engineer, Gladiator, Graveborn, Grovetender, Mime, Paladin, Purgatorian, Rune Knight, Shadowbringer, Siegemaster, Spellweaver, Toxicologist, Tribalist, Trickster, Windrunner.

This guide isn't about the advanced specs so I'll keep this EXTRA brief:
-Astrologer likes having a good number of buffs & minions, & starting with a bang (you go first)
-Brewmaster loves maximum chaos, kinda like cabalist, though less tanky
-Demonologist loves mostly belphegor & asmodeus (hellknight-in-a-can minion) & abusing start of battle actions like hotspur into panic attack etc (because greaters can disappear start-of-turn)
-Doombringer loves triggering EVERYTHING off of EVERYTHING constantly (consider phoenixes)
-Dreamshade is like cleric (has overheal perk, but it's called "paradox"), +random effect goofery
-Druid uses just 4 creatures & main parents from races with strictly positive traits (e.g. sparktails)
-Engineer loves having high base speed on one ally & many traits that say you 'have' more speed
-Gladiator likes smiths & weird aritfact stuff, mostly (chance to snare on trick slot + AoEs especially)
-Graveborn loves big stats and glass cannons mostly
-Grovetender is a worse, weirder, spammier version of animator; still better than sorcerer
-Mime likes reacting to enemy actions by copying them (post-story: Sarea's Command is NEAT)
-Paladin likes taking damage and not dying (sparktails are redundant btw, don't use em)
-Purgatorian likes having its team be mostly dead save for one or two allies that refuse to die
-Rune Knight loves stacking 1 stat HIGH & spamming rune spells +on-attack things (e.g. hemomancers)
-Shadowbringer loves healing enemies to death (use Sea Shamblers and NOT life shapeshifter)
-Siegemaster loves having absurdly high defense (it's all you need)
-Spellweaver loves spamming the devil out of various spells of various classes (needs turns)
-Toxicologist loves having high base ATK & effects that make you 'have' more ATK (& Smogs)
-Tribalist uses strong race synergy & big stats & basically whatever it wants
-Trickster is weird, kinda likes attacks, a bit all over the place (maybe consult Discord...)
-Windrunner loves things that say your creatures 'have' more speed... SPEEEEEEEDDDD

Special ones:
-Deprived & Pariah are "challenge specializations"; they're made to give late-game players a build challenge if they want it, and the reward for taking on this challenge for a certain late game battle gate is a boost to skin droprate (deprived) & nether stone droprate (pariah).
-Fanatic is something based around usage of "avatar" and "godspawn" race creatures. The former is basically the lesser form of a god, and the other is a special creature corresponding to them that lore-wise was supposedly born in their realm (but also many wandering out), and you unlock a project to get said godspawn's mana after reaching rank 20 with their respective god. This ironically makes them more of a pain to collect in the late game than the avatars themselves...
-Royal requires all other specializations to be unlocked to use, and effectively has no perks at all until you reach depth 415+ and unlock guilds to start earning annointments. Annointments are basically perks that come from other specializations, and you unlock the ability to use them on other specializations, up to 5 annointments on each specialization. Royal's perks are to let it use up to 15, or 18 if ascended, or 20 if ALL specializations are ascended.

IMPORTANT NOTE:
-Brewmaster requires buying an item from the dwarf shop (you place one in your castle, and talk to them; have to play their games to earn notoriety, so if you need tickets go to Gambler's Hive AKA one of the realms you unlock via project to earn loads compared to other realms).
-Gladiator requires buying an item from the arena shop (another thing you place in your castle)