Recently I completed a solo playthrough using a dragon's disciple, and thought it was very fun and rewarding. When I did my research on spell choices, many spell recommendations were either based on HoF difficulty, based on BG1/2 series, or weren't very thorough. Hopefully this guide will aid others who's also looking to do the same.

While I only did my playthrough in core difficulty, it was almost too easy. In retrospect, perhaps I should've done it on insane. Although, I'd argue that insane may even be easier, as you get more experience early game, so you level faster in early game. Late game would be harder (especially for a few mage fights), but should be quite manageable as we're simply too powerful.

As for HoF difficulty, personally I don't like it at all, as the monster's health is way too much, and it leads to a summon-only play style that is just not fun for me.

I mostly talk about spell choices here. If there is interest, I may talk about a few specific gameplay tips.

Class choice: Dragon Disciple. Simply way better than the vanilla sorcerer. Main advantages are: 1) Much higher AC; 2) Much higher hp; 3) fire immunity and late game healing from fire; 4) Dragon breath ability helps out early game a lot. The loss of 1 spell at each level is very minor.

I recommend being good alignment, so you can wear Shimmering Sash from Orrick. If not, consider taking Blur and drop perhaps Glitterdust in your level 2 spells.

Starting stat:
Strength: 18
Dexterity: 18
Constitution: 16 (enough) or 18 (for late game regen)
Intelligence: 9+
Wisdom: Rest
Charisma: 16+

Among these stats, dex is the most important as AC is very critical to not die. Strength is important for early game (so your melee attack kill them faster) and ultra late game (black blade + improved haste etc have higher damage/Thaco). Con 16 is mostly enough, but as a DD, if you start with 18 cons, late game you get to 20 and can start to regen health. Charisma is mainly for better store prices early on. Intelligence and Wisdom doesn't matter all that much really. Wisdom is only useful for late game level 9 Wish spell, and we can chug wisdom potion for it.

I rolled a 92, so I went 18/18/18/11/11/16. But if you really hate rolling, doing something like 18/18/16/9/3/16 (80 points total) is probably just fine.

Proficiency:
Sling -> Dagger -> Quarterstaff -> Dart

In this game, mobs try to swarm you all the time, so in general we should prioritize AOE spells over single target spells. Also, enemy mages are not very strong, so magic battle spells (spell trap, spellstrike, ruby ray) are quite bad here.

All spells below are listed in the order that I suggest you pick them.

Level 1:
Shield
Sleep
Magic Missile
Identify
Protection from Evil

Shield gives you base AC 4, a +6 improvement v.s. base line, and is extremely important to keep you alive early game. Once you get necromancer's robe (AC 6) in dragon's eye, this becomes less important. It also gives you immunity to magic missile which is quite helpful in many encounters.

Sleep is only used for prologue. But it's insanely strong at that. One can argue that prologue (orc cave and kuldahar pass) is the most difficult part of the game for a solo sorc, so it's worth investing a level 1 spell to get through that. Note that while a mob is asleep, your hits are 100% going to land. You can even use a quarterstaff that you're not proficient of, for maximum damage.

Magic missile is your main damage spell to kill early game mobs (other than swinging your weapon). Its damage is quite pathetic at level 1 so we take it at level 3.

The other spells are not as critical and feel free to try others.

Identify is super convenient for solo, without it there's a lot of running back and forth just to ID items, a hassle that I just don't want to deal with.

Protection from Evil basically flat out gives you +2 AC and +2 saving throws, which is useful through out the entire game.

Honorable mentions:

Grease: I find the AOE of this spell too small (6 ft). I also think crowd controlling enemies is not as good as in BG, just due to the sheer amount of enemies present. But perhaps it could suit your playstyle.

Chromatic orb: While powerful, it has two main disadvantages: 1) it's has no saving penalty; 2) even if it lands, it's single target. When you constantly get swarmed by 10+ mobs, the last thing I want to do is spend my entire turn to throw out this spell that disables one guy and will most likely fail.

Spook: This one has much better save, but 1) it does not work on undead; 2) it's also single target. IWD has way too many undead enemies so I find this also not great.

Burning Hands: technically AOE but I find it hard to land on multiple enemies. Can be your troll solution too.

Charm Person: mainly used in HoF to get your early summons.

Color Spray: I would pick it over sleep if it's not bugged. I hit a mob and it would wake up immediately.

Level 2:
Invisibility
Mirror Image
Knock
Web
Glitterdust

Invisibility is extremely important. Before you rest you must first cast invisibility. This way, even if get woken up by mobs, you can easily get out of sight and rest again. Note that it lasts for 24 hours, so if you only rest for 8 hours, after rest you're still invis and can proceed to scout a bit. Also, any defensive spells cast won't break your invis.

Mirror image is 2nd most important defensive spell in your arsenal (right after stoneskin). It negates physical damage and many trap damage. Keep it on pretty much always.

Knock is essential for a solo run.

Web is insanely strong in BG1/2, but here, due to the much smaller radius, it's not as good. Still very potent, but I tend to use it less and less later on. Especially some late game enemies are immune to it.

Glitterdust is quite strong as it's 1) AOE; 2) saves with -4 penalty. So it tends to disable 30% ~ 50% of the enemies you face. It's not a must-have but quite helpful against enemies that can hit you hard. Note that blind also basically disables enemy casters.

Honorable mentions:

Aganazar's Scorchers: while it's useless late game, before you get level 3 spells, it can be a decent AOE option.

Snilloc's Snow Swarm: another early AoE option, but I think cold damage is not great when many mobs have cold resistance in this game.

Blur: Very good but not needed for me if you're good-aligned and can get Shimmering Sash from Orrick.

Resist Fear: Take this if you don't pick Mind Blank level 8.

Level 3:
Skull Trap
Melf's Minute Meteors
Slow
Dispel Magic
Protection from Cold

Skull trap: your main damage source for the game, and it's way better than fireball. Fireball caps out at level 10 (10d6 damage), while skull trap goes all the way to 30d6.

MMM: Use it to finish off enemies not killed by your AOE spells. Very helpful for killing Chapter 2 boss.

Slow: Probably the best mass enemy disable: doesn't hit your party, -4 penalty to save, and lasts for a long time.

Dispel magic: in this game, the success chance of this spell is 100%, so it's very very strong, even though it can hit yourself. This is also the spell that you fear the most from enemy mages.

Protection from Cold: not super critical, but very useful as there are so many enemies with ice-based attacks. Casting this in the Heart of Winter final boss fight just renders the boss kind of useless.

Honorable mentions:

Fireball / Protection from Fire: There's a combo when you cast both, your fireball actually heals yourself. I tried it and it wasn't that impressive for me. Once you get stoneskin, you rarely get hit anyway, and it doesn't heal that much. Spending two slots for this niche combo is simply not worth it in my opinion.

Remove magic: it doesn't hit yourself, but with the price of not guaranteed success. Also, if you decide to go with Tenser's transformation, you won't be able to "free" yourself from tenser's.

Level 4:
Stoneskin
Vitriolic Sphere
Improved Invisibility
Wizard Eye
Mordenkainen's Force missiles

Stoneskin: Keep it on at all times.

Vitriolic Sphere: Not a great choice, but level 4 spells are overall not that strong, so I think it's fine to leave it here. If you do decide to take it, take it 2nd as it gives you another early damage option. On paper it can deal damage multiple times, but most enemies tend to save against it, so you just deal a one-time 20~40 single target damage. If you don't take this, take Force missiles here instead. It's worse early game but much better late game.

improved Invisibility: This is the ultra defense against enemy mages. Even if you break invis, they cannot target you, and typically just stand there and do nothing (unless they dispel or cast something like True seeing). The +4 to AC and saving throws is just the sweet sweet cherry on top.

Wizard Eye: Important later on when you get project image, so it travels with you and help your project image wipe the map.

Force missiles: Very good late game single target damage, can be put in level 7 spell sequencer.

Honorable mentions:

Dimension Door: Your "oh sh*t" button to get out of enemies that surround you.

Greater Malison: It's only -2 to saving throws, as opposed to -4 in BG. It's better as a support spell to help other casters, and less good in a solo playthrough.

Spirit Armor: AC 1 is strong, but it doesn't last very long and you lose life afterwards. In Chapter 6 you can buy a pair of gloves with AC 2, so I don't think this spell is that critical.

Minor sequencer: only houses two spells, and only up to level 2, basically either double magic missile or double web. Not super appealing.

Emotion Hope/Emotion Courage: Great spells to assist your team doing physical damage, but not for a solo run. In late game, you can just let your simulacrum cast this from the quick item slot.

Emotion Hopelessness: would be strong if the radius (7 ft) isn't that small.

Level 5:
Sunfire
Spell Immunity
Lower Resistance
Protection from Electricity
???

Level 5 spells aren't super great, so there are quite a few options here.

Sunfire: Great for solo runs, damage caps out at 15d6 and has a higher AoE range than skull trap. This and skull trap will be your two main sources of damage until you get incendiary cloud.

Spell Immunity: with Abjuration, this is the only spell in the whole game that protects you from dispel magic, which enemy priests love to cast. The only other protection against it is improved invisibility.

Lower Resistance: essential as many late game enemies have a lot of magic resistances. The earliest enemy with resistance is the Chapter 2 boss, so if you're concerned, consider getting this earlier.

Protection from Electricity: the rest of the spell books are all meh to me, so I picked this to avoid trap damage, and the occasional lightning bolt spells.

5th slot: anything you prefer in the following list.

Honorable mentions:

Breach: S tier spell in BG but not that great here. Enemy mages simply don't have much protection and even if they do, you can just do dispel magic.

Chaos: With -4 penalty to saving throws, this one hits a lot. But 1) I hate mobs running all over the map, and chasing them can lead me into undesirable positions; 2) Level 6 spell Shades summon Umber Hulks that do this too.

Cloudkill: Can be fine against enemy mages but then there aren't that many of them, this hits you too, so it can make your fight awkward.

Conjure Fire (or others) Elemental: A fine summon to have. If in HoF then do this.

Feeblemind: It's a mini-finger of death if you land, and hits undead too. Downside is it's single target, and it still fails a lot.

Spell Shield: Late game in trials of the lure master, quite a few enemies cast Ruby Ray, and this can be an ok protection against it. But we can just do improved invis or spell turning.

Level 6:
Protection from Magical Energy
Shades
Improved Haste
Disintegrate
Tenser's Transformation

Protection from Magical Energy: safe to cast skull trap anywhere now.

Shades: Decent summons that can tank even in end game. Umber hulk confuses enemies, and trolls can't die and can tank indefinitely.

Improved Haste: Helps us go ham as a melee fighter late game.

Disintegrate: a good instant kill, just watch out and don't kill targets that may drop something you want. Late game you're so powerful and you can just skip all loots, and this is not that much of a down side. It's a single target spell so not super critical either.

Tenser's Transformation: I really struggle whether to use this spell. When you use it, you do become a decent fighter (with good Thaco and +1 attack), but you can't cast spells for a long long time, and this can end up getting you killed. You can either only let your simulacrum cast it, or when you decide to cast it on your main body, let the simulacrum be prepared to dispel you if things get out of control. Overall I think it's a net positive. Remember to do all your buffs beforehand.

Honorable mentions:

Death fog: a stronger cloud kill, with the same issues.

Death spell: while S tier in BG, the spell works differently here and is quite bad.

Power word: Silence: while strong, it only lasts 2 rounds.

Protection from Magical Weapons: Stoneskin is enough.

Level 7:
Project Image
Spell Turning
Spell Sequencer
Mass Invisibility

Project Image: Now you become a god. Cast this and cast wizard eye and you can have your image clear half of the map, while only spending 1 level 7 spell slot.

Spell Turning: Not only does it absorb multiple spells, it reflects them onto the enemy! It's quite funny to see enemies breach themselves.

Spell Sequencer: I typically store 3 skull traps or 3 force missiles in it for instant damage. I don't use it that often though, almost feels like cheating when I do.

Mass Invis: optional slot here. It helps your Shades get better AC. Or you can put it into a chain contingency to save yourself when disabled.

Honorable mentions:

Cacofiend: I don't like these devil creatures as their kills don't count as mine, but definitely can try for them if it fits you. Requires Protection from Evil.

Finger of Death: I heard it's bugged and it also destroys items, same as Disintegrate. So beware.

Prismatic Spray / Sphere of Chaos: these two spells are both flashy, and have a lot of random effects. Not my preference but certainly can try them. I like the spray over sphere here.

Power word: Stun. Single target, and it only hits enemies with <90 hp. Late game enemies mostly have more HP than this so I find it too restrictive.

Level 8:
Incendiary Cloud
Mind Blank
Power word: Blind
Simulacrum

Incendiary Cloud: an insane spell that does the most damage in this game, works exceptionally with Dragon Disciple as you can just heal from the Sauna standing in it.

Mind Blank: One stop shop to prevent all possible disables, lasts for 24 hours too.

Power word: Blind: unconditional instant disable, and it has a small AoE. It lasts for 1 turn, not super long but should be enough for most hard fights.

Simulacrum: Your ultimate late game friend that kills with you side-by-side. It's 60% of your level, same as in BG, as opposed to the 50% in the description. So it gets all level 9 spells when you're at max level 30. Give it wisdom potion, give it emotion hope/courage, give it all the scrolls that you want to cast but didn't pick, and let it blast for you.

Honorable mentions:

Horrid Wilting: It does a lot of damage, but only one-time, as opposed to the lasting effect from incendiary cloud. It also requires Protection from Magical Energy as you can hit yourself with it.

Spell trigger: comparing to Spell sequencer, this allows level 5/6 spells. But the problem is there aren't many spells at those levels that I want to spam.

Symbol Stun: strong but still requires saving throws. I like power word blind more.

Level 9:
Timestop
Black Blade of Disaster
Chain contingency
Wish

Timestop: Need I explain?

Black Blade of Disaster: Turns you into a semi F/M. Good solution to many late game enemies that are hard to kill with magic.

Chain contingency: use it either as a "oh sh*t" button to save yourself while disabled, or offensively to throw damage spells at enemies when you see them. Flexible and strong.

Wish: Together with wisdom potion, helps you fish for the instant rest option. I always let my project image or simulacrum cast it.

Honorable mentions:

Gate: Ultimate devil if you're into these types of summons.

Shapechange: can be fun to turn into a mind flayer etc, never tried it myself.

Wail of the Banshee: Decent AoE instant death. I just let my simlacrum cast it with scrolls though.

End-game gears:

Main hand: Mage Dagger +4
Off hand: Fire dagger +2 or Fang of the Gloomfrost +4
Chest: Robe of Enfusing
Bracers: Silver Bracers of Kedl (AC 2)
Sune's Laurel of Favor: (+1 AC, +1 Charisma)
Kossuth's Blood: (-2 Cast time)
Cloak: Cloak of Protection +2
Rings: Kontik's Ring of Wizardry (double level 1, 2 spells)
Edion's Ring of Wizardry (double level 5 spells)
Boots: Boots of Speed
Belt: Shimmering Sash

Without any buffs, at the end I have
19 strength (from potion that gives permanent +1 Strength, -1 Constitution)
AC: -17 (-18 with Fire dagger)
Single weapon Thaco: 6
Dual wield Thaco: 10 / 14

Among these gears, unfortunately some of them are random drops: Mage Dagger+4, Fire Dagger +2, and the potion. Mage dagger+4 is the most critical one and I had to roll for it. Fire dagger just offers another AC point and can be replaced by other Mage daggers. The +1 Str -1 Con potion is from Kerish, and I just got lucky. You can also consider rolling for the Ring of Strength in the 3rd floor in Tower of Solonor.

When you're not trying to fight melee, put on dual wield for maximum AC benefit. When you are trying to melee, weigh in if you want the two extra attacks (with improved haste) v.s. if you prefer better thaco. In a time stop, all swings are guaranteed to hit so you must put on both weapons.

If you didn't get any daggers with AC, then your end game AC will be "only" -15. With buffs, you can still be almost impossible to hit other than them rolling a 20. If you didn't get any +1 strength equipment, then you'd have to work with 18 strength, as opposed to 19, this does make going ham as a melee fighter in late game much harder. Luckily Orrick sells a robe that gives you 18/99 strength and +2 Constitution, and you can wear that instead.