Iudex Gundyr is the first boss and can be very hard for beginners. It takes practice and learning how to roll his attacks, but experienced players will usually parry him. Iudex Gundyr is a generally easy boss to parry because his attacks are so slow and delayed most of the time, but keep in mind that not every attack can be parried! You can only perform parries with parrying equipment, for example small and medium shields, parry tools and other exceptions which you can see for yourself on the wiki.
Back to the point, the fight is fairly easy up until phase 2.
In Phase 2, he is very weak to fire, meaning you should use the firebombs you found just outside the boss arena. Best is to keep your distance and throw bombs, careful not to miss, they are limited. His attacks can be both quick and fast so it takes some practice as well, but he should die in 5 firebombs and a couple swings. This boss is very easy if you start as a pyromancer.

He is weak to: Frostbite, Lightning, Fire
He is resistant to: Dark, Bleed
He is immune to: Poison, Toxic.

Vordt may seem scary at first because of his fast charge attacks, but he is very easy if you keep in mind this one trick: Stay right behind him and keep swinging at his back, and you really want to make sure you just consistently and continuously stay behind his back.
However, when phase 2 starts this trick won't work because he will charge you about 4 times. Just roll his charges at the right time and get back behind him and keep swinging.

Weakness: Dark damage
Resistance: Magic and most physical damage, but this won't matter; you're still gonna deal good damage to him.
Immunity: Frostbite, Bleed, Poison & Toxic.

Recommended weapon: You might wanna use the Deep battle axe you acquired from defeating the mimic where the wyvern/dragon spouts fire at you near the Tower on the wall bonfire. Google the mimic location at Tower on the wall if you can't find the place.
The Deep battle axe significantly increases your damage per minute against Vordt because he's weak to dark damage. This damage is good enough to nullify the Physical damage resistance.

The Curse-rotted Greatwood is an easy enough boss and quite straight forward. Walk into the boss arena and kill the fodder enemies laying around, he will awaken when you get close enough to him.
Just hit his sacks until they burst and he should die pretty quickly. His sacks are usually located at the front, at the back, on his arms and legs. The sacks usually respawn in the second phase.
More importantly, in the second phase if you can't find the last sacks you need to burst, you can attack his Hand and deal normal damage. This is however very risky as he tends to spam the grab attack and constantly grabs you. When you see the hand is moving and then stops for a second, roll his grab and then keep attacking the hand. The hand is especially vulnerable when you burst a sack.

He is weak to: Slash, Thrust and Fire damage.
He is immune to: Poison, Toxic, Bleed and Frost.

*Health value is high because his sacks have very high health.

The crystal sage can be a very annoying boss to deal with due to the constant spell spam in the second phase and the fact that the nearest bonfire is quite far from the boss room. If you get too close to him, he will swing at you with his dagger. This can be parried but it is risky as it's a pretty quick attack. His spells do very high damage, so you need to keep an eye out and watch all your angles to avoid getting spell spammed. This is harder said than done...
In second phase, the most important tip for this boss is to throw throwing knives at the clones he summons. Any hit dealt to the clones will immediately kill them. After that, you can run to the main Sage and stagger him with r1's. Take the time while he teleports to buff your weapon or use Estus.

This boss is weak to: Lightning, Fire, Dark, Poison, Toxic, Frostbite.
Resistant to: Magic damage, but not immune to it.

NG+ tips: If you acquired the spell Vow of Silence from Irina of Carim, you can use that and the Crystal Sage will not be able to use any spells for 60 seconds.

This boss is incredibly easy. The only danger is getting swarmed by too many of them or more importantly when the Archdeacon starts summoning a curse on you in phase 2. If you don't interrupt the archdeacon, your curse meter will instantly fill up resulting in immediate death. You can throw Alluring Skulls around the arena and they will be busy chasing after them.

This boss is weak to: Frostbite
Resistant to: Dark, Magic, Bleed, Poison and Toxic.

Recommended weapons: Weapons with wide swings are useful for this boss, for example most greatswords and the great club.

NG+ tips: You can use Lightning Storm to wipe all of them out quickly or Vow of Silence to cancel their fireball spam.

This boss is the first lord of cinder and might be the biggest challenge so far. It's very important to go for critical attacks against this boss, such as Backstabs and parries. Backstabs are easier to pull off so I recommend doing those a lot. He has very long combos in phase two, so you need to keep your distance and only get near him when he finishes his combo, that way you can backstab him. Other than that, it's mostly practice.

Weak to: Lightning.
Resistant to: Dark and Bleed.
Immune to: Poison, Toxic.

Recommended weapons: Try using weapons that can stagger him and swing quickly, for example shortswords or twinblades. Greatswords will do very well too.

The Old Demon King is an optional boss in the Smouldering Lake. This boss can be tough at first. He will usually either have a long attack combo or use Great Combustion on you if you're too close. You will constantly lose lock-on on this boss most of the time and he will summon fire circles on the ground that close in on you. Stay close to him and keep swinging, he should go down eventually.
When he's at about 1/4 health, he should stop attacking for a bit and charge up. You can go up to him and kill him if your damage output is enough and he's low on health and you have time. After charging up for about 3 or 4 seconds, he will explode. This explosion will most of the time kill you in one hit and has a massive area of effect. You can roll this explosion, but I don't know if you can out-distance it. After his explosion, he's vulnerable for attacks.

He is weak to: Dark.
He is resistant to: Fire, Poison, Toxic.
He is immune to: Bleed, Frostbite.

Recommended weapon: If you have a weapon long enough to reach up to his head, you can hit it and it should stagger him in about 4 or 5 hits depending on your damage output.
Black knight weapons are also useful as he is a Demon type enemy, though you might not have those yet because this is pre-untended graves. The only way you could get a Black Knight Greatsword is by farming the guy in Crucifixion woods where the Farron Coal is or you can use the Black Knight Sword (not the greatsword) which you find in the Demon Ruins.

This is also a very easy boss. All you have to do is hit his 3 bracelets and he will die. The biggest danger is staying in the purple mist, which will kill you very quickly. He might charge forward many times and force you into his purple mist, so keep your guard and distance. If you don't kill him quickly enough, he might push you all the way to the back of the arena and slowly kill you with the mist. When he spits his mist out, you might wanna run, however the area where his arms are will not be affected by the mist, so you can stay in that small tight spot and swing at his bracelets.

Weak to: Strike damage and blessed weapons.
Resistant to: Dark, Poison and Toxic.
Immune to: Bleed and Frost.

*His health is high because the bracelets count as health.

This is a very challenging and tough boss, he has very long combos and massive reach. When the fight begins, he will charge forward at you. This is your opportunity to parry that attack and deal a big chunk of damage. Pontiff can be parried very often albeit with difficulty and requires practice, you cannot backstab him however. A shield or a greatshield helps greatly in this boss fight as you can block most of his attacks, if you have enough stamina of course. Parrying his attacks is important because you can riposte him for 1.5k damage with nearly any weapon I think, and just general practice. This will be hard, but don't beat yourself up, parrying timing can be hard and he has many different attacks. Therefore you can either try parrying him or just swing and roll. If you decide to swing and roll, sticking to his right side will help greatly as he should miss most of his attacks that way, because of the height difference between him and your character. He is vulnerable when he summons his clone and you can deal some extra damage on him when he starts his second phase, he will damage you a bit if you stay near him when he does that but I think it's personally worth it. Try killing his clone as quickly as possible and try playing passively in 2nd phase, if you're too close you will get r1 spammed by both of them. Good luck!

He is weak to: Fire, Lightning Dark, Critical hits.
Resistant to: Poison, Toxic, Bleed and Frostbite.

Recommended weapons: You should use quick weapons since he has very quick and dangerous attacks. You can stagger this boss, but it's not recommended because it's very risky and you need massive damage output for it. Quick weapons let you roll earlier and consume much less stamina, however greatswords will work fine on this boss fight too, as long as you don't get greedy. Greatswords are probably better if you want to focus more on parries, because of the greater damage output.

Aldrich is the second Lord of Cinder and he's relatively easy compared to Pontiff. Hitting his "human" body will deal more damage instead of hitting the writhing part of his body. You can also use Vow of Silence to stop him from using some of his spells. His bow and arrow attack is very dangerous, but it's fairly easy to deal with. The arrows seem to be homing in on you in the first phase, so try running in a circle to throw them off. In the second phase, they will last longer and are more dangerous because they home in on you more. To avoid getting hit, either go forward ahead of the arrows or spam roll/try rolling in circles to throw off the homing. The rest of his attacks are pretty slow and delayed apart from the purple ball he throws at you with his spear sometimes. He does however have a quicker attack like the spear charges or the lifehunt scythe he uses.

Weak to: Fire, Lightning, Wolf Knight/Farron Greatswords.
Resistant to: Magic, Dark, Frost, Bleed.
Immune to: Poison and Toxic damage.

Yhorm is the third lord of cinder and probably the easiest. You will notice weapons don't deal much damage to him, that's because you need to run to the back of the arena and pick up the Stormruler, charge up the weapon art and then use it to deal massive damage. You need to be careful in second phase because he can lunge at you or attack you suddenly, resulting in you not charging up the Stormruler quickly enough and he deals massive damage. This boss fight is probably the easiest in the entire game if you summon someone to help you.

Use the Stormruler!

*He has very high health because the Stormruler's weapon art deals massive damage.

I forgot about this boss when I first made the guide, but for a good reason. This boss is probably the easiest boss in the game next to Yhorm. It's a nice twist on boss fights by FromSoftware but it didn't really work out too well in my opinion.
Anyway, the entire boss fight you have to run all the way up to an area where you can perform a plunge attack on the Ancient Wyvern. You can also kill the snakemen but this will take time and can be hard because they're tough enemies. You can also kill the Wyvern normally on ground, but it will take a very long time.
Once you're up at the point where you can perform the plunge attack, make sure you wait for the Wyvern to stop doing his fire breath attack and when he stays idle, jump and plunge attack. Sometimes you might fall and die, so if you have SIlvercat ring you can equip that if you don't want to lose souls/ember/pride.

Weak to: Lightning, Frostbite.
Resistant to: Poison, Fire, Toxic, Bleed and most physical attacks.

NG+ Tips: You can try doing a glitch where you go behind where the wyvern spawns and perform a plunge attack off a small ledge when he's facing you. This can be tricky, but do-able. It's an easy way to cheese an annoying boss, if you want to try this, feel free to google it.

This boss is considered one of the hardest bosses in the game, albeit optional he is worth killing in my opinion. His first phase is quite easy, just target the wyvern's head and keep an eye out on the King, as he will usually smack you with his spear. Once you stagger the wyvern, it dies after the riposte most of the time.
The King of the Storm may sometimes fly up in the air and breathe fire in a very large radius on the ground, so look out as this catches many players off guard.
Phase 2 is where the fight begins. He will have long range attacks and very delayed attacks, mixed with fast attacks too. It takes practice and reflexes to beat this boss, it's a true challenge on first playthroughs, let alone NG+3 or higher.
If you hit him enough times with enough damage output, you can stagger him. If you really struggle with this boss, summon a mage. He is very weak to magic. You can also use a shield that has lightning damage resistance if you're struggling a lot.

Wyvern is weak to: Lightning, Dark, Frostbite
Nameless king is weak to: Fire, Dark
Wyvern is resistant to: Fire, Bleed, Poison & Toxic
Nameless king is resistant to: Lightning, Frost, Bleed.
Nameless king is immune to: Poison and Toxic.

Recommended weapons: As mentioned before, try using a lightning resistant shield. You should use quicker weapons against Nameless King.

*King of the Storm

*Nameless King

This boss can be very challenging at times. She will attempt to grab you often in the first phase, so keep your distance. She also has very long combos that deal massive damage, so again: Keep your distance and more importantly don't get greedy with attacks, if you run out of stamina she has enough range to reach you most of the time.
When the second phase begins, she will start doing a very long spin-combo, you need to stay far away. She has very long combos in second phase as well.

She is weak to: Dark, Lightning, Bleed.
She is immune to: Poison, Toxic and Frostbite.

Recommended weapons: Anything that doesn't use up a lot of stamina.

This boss is an easy optional boss guarding the entrance to the Untended Graves. Most of his attacks, he will charge at you and just hit the walls instead, but if he hits you he will deal some pretty big damage. You also wanna stay away from the mist he summons sometimes. Other than that, this boss mostly dodges its attacks for you, literally. His attacks are sudden though, so be ready.

He is weak to: Lightning, Frost
He is resistant to: Magic, Bleed
He is immune to: Poison and Toxic

This boss can be very challenging if you don't know how to fight him. His first phase is relatively easy, the biggest and most important tip for this boss: Parry his attacks! Just like Iudex Gundyr, his attacks are slow and heavy, so parries are the best way to fight this boss.
You should really not try fighting this boss head-on without parries in second phase. His stamina is endless, he does a lot of damage, has a lot of range and will catch you off guard with Karate moves all the time.

He is weak to: Lightning, Frost and Bleed.
He is resistant to: Poison and Toxic
He isn't immune to anything.

Recommended weapons: Something with very high damage for parries.

This boss will hit you very hard and do massive damage most of the time. You can get a few extra hits in when he does his lightning strike attack or he gets staggered, but rolling them is top priority as they might one-shot you. Stay far from the edge of the bridge, as he can sometimes knock you off. His second phase you need to be careful as he has angels on his side that will hinder you, so watch the ground and the sides for things flying at you.

Weak to: Frostbite, Wolf Knight and Farron Greatswords.
Resistant to: Dark, Lightning.
Immune to: Poison, Toxic and Bleed.

NG+ tip: You can cheese this boss fight by using the Snap Freeze or Pestilent Mercury spells which deal masive damage. Snap Freeze is preferred because this boss is very weak to frostbite.

Recommended weapon: The Irythill straightsword or Vordt's Greathammer can be good for this boss fight as he is very weak to frostbite. The straightsword is preferable because it uses less stamina and attacks faster, therefore you can roll away quicker if you have to.

This boss hits really hard and you might have a hard time dodging the attacks. When Lorian charges up his holy blade, roll just after it hits the ground.
Big tip in second phase: You should try attacking Lorian's back, as you can hit Lothric and deal extra damage that way. Don't get greedy with this boss, usually do 1 swing with greatswords and 2 or 3 with shortswords, maybe more if you get good RNG on their attack patterns.
Lothric will usually shoot a bolt of magic at you very quickly which deals much damage. This will catch you off guard often, so keep an eye on him. Dodging Lorian's attacks is mostly reflexes and knowing the attack patterns.

Both princes are weak to: Frostbite and Bleed
Lorian is weak to: Lightning and magic
Both are resistant to dark damage.

Recommended weapon: As this boss is weak to frostbite, the Irythill straightsword is also very useful here.

The last boss in the base game excluding the DLC. This boss fight can be hard at times, when he summons his Curved Sword/Scimitar and his Spear. In second phase, he will usually make this stance where he preps his sword and then swings several times and ends it with a big explosion, keep your distance.
His normal phase: Just roll his sword attacks
His scimitar phase: This phase is very hard, roll his attacks and don't get greedy
His spear phase: This can be tough, stay close to him and wait for him to attack. Again, don't get greedy.
His mage phase: You can get greedy here as long as you dodge his flash sword attack. Stay close to him so that the spells are uneffective.

Weak to: Lightning, Dark.
Resistant to: Fire, Bleed
Immune to: Poison, Toxic.

Recommended Weapons: Any Greatsword/Curved Greatsword or hammer can be good for this fight, as you will occasionally stagger him allowing you to get a bunch of extra hits.

This boss can be challenging at times, there isn't much to be said. When you first start the fight, kill the 3 wolves and r1 spam the gravetender. He might parry you. You can also backstab or parry him. He can shield attacks when he charges at you sometimes because of the weapon he uses, Valorheart which he also drops. At half health, he will summon the greatwolf. Try killing him quickly so that you can fight the wolf on your own. The Wolf will have very hard to dodge charge attacks and deals much damage. You can stagger him if you hit his head enough.

Weak to: Bleed, Fire, Lightning
Resistant to: Frost, Poison/Toxic

Recommended weapons: This boss is very weak to bleed, so try using a weapon that causes bleed if you can.

Sister Friede can be very tough for new players because she has 3 phases. A big tip is practicing the first phase and using the least amount of Estus. Another important tip is, when she goes invisible, you need to listen to the audio and look around you to see where she will teleport. Run at her and swing at her so she won't do her charge attack.
In Phase 2, you should focus on Father Ariandel as his attacks are quite easy to dodge most of the time, but watch out for his big fire smash attack which will deal massive damage and has very big range. He will get staggered very often and Friede will try healing both sometimes, so run towards her and go for a backstab.
In Phase 3, she will begin with an attack where she goes up in the air and slams down releasing a black flame explosion, along with a straight forward black flame attack. To dodge this, run towards her as she slams into the ground and get behind her, then backstab. The rest is mostly practice.
You can try parrying her, this is made very easy with a parry shield: Block her scythe combo's first/second attacks and then parry. I'm not sure about this as I never did it before, so feel free to look up "Friede parry".
You can also go for backstabs a lot in this boss fight.

Friede is weak to: Hollowslayer Greatsword, Bleed, Lightning, Dark damage.
Ariandel is weak to: All damage.
Both are immune to: Frostbite.

Recommended Weapons: If you have the Ringed Knight Paired Greatswords from The Ringed City, you can use the long attack combo and stun lock her and deal massive damage. You can also alternatively use the Winged Knight Twinaxes's weapon art if you have a summon. This will stun lock her for your entire stamina and FP bar, but will allow your summons to get a lot of damage in.
I can't stress this enough, the RKPG makes this boss fight a joke because of the long stagger combo.

*In phase 2, Friede and Ariandel's HP is mixed together.

This boss fight can potentially be incredibly hard if you don't use any summons such as Slave Knight Gael or Amnesiac Lapp. Online phantoms work too as long as they don't immediately die.
This boss is very hard because of the massive health pool, long combo attacks and high damage.
You should first target the Demon from Below, as this will cause the second phase to have an attack which charges up homing fireballs. If you hit the Demon Prince quickly enough during this, it will stop the spell and the Fireballs will simply crash on the ground (meaning you should run away after staggering him), therefore not homing in on you.
You should really summon someone for this boss fight, dealing with both of them at the same time in phase one can be very tough.
This boss has a very, very big health pool, so save your Estus.

Weak to: Bleed and Black Knight Weapons, as they deal more damage to demons.
Resistant to: Fire, Magic, Lightning
Immune to: Frostbite, Poison, Toxic
*This boss will get staggered many times both in first and second phase if you hit their heads. Therefore, you should use the Hornet ring for extra riposte damage.

Weapon Recommendations: Any Black Knight Weapon will deal extra damage.

NG+ Tips: I'm not sure how well this works for NG+2 to NG+5, but on NG+6 or higher, Slave Knight Gael and Amnesiac Lapp will deal massive damage to them as their damage scales. Slave Knight Gael can usually deal about 3000 damage in like 1 minute, basically matching your damage and making this boss fight extremely easy. He will however die as NPCs don't always roll attacks.
So make sure you summon Gael, Lapp or both for this fight if you're doing any NG+

*Both demons have the same amount of HP in first phase.

Game Cycle

1st Phase

2nd Phase

Total Health

NG

7045 x2

7045

21135

NG+

7151 x2

7151

21453

NG++

7866 x2

7866

23598

NG+3

8223 x2

8223

24669

NG+4

8581 x2

8581

25743

NG+5

9296 x2

9296

27888

NG+6

9654 x2

9654

28962

NG+7

10011 x2

10011

30033

This boss is very feared because he deals massive damage, has a very big health pool, hard attacks to dodge, long combos, a second phase, great reach. You can riposte Midir if he reaches around 1/4 Health. Usually he lives with a sliver of health, so run up to him and kill him.
The biggest mistake people do when fighting Midir is hitting his legs, tail and arms. Do not do this!
Target Midir's head instead as he takes way more damage and shortens the fight greatly.
As for his laser attacks, in first phase try standing far away enough from him so the fire doesn't hit you, a bit to his right. The laser will always go in the same spots. Once the laser shoots straight and the fire is gone, run up to his head. He shouldn't be able to hit you there as he shoots his laser further and then you can get some hits in. When he goes into second phase, he will have a new soul attack. He will launch souls at you which you can dodge by simply running away or roll-spamming. His second phase's laser attack can be very dangerous and drags out for like 15 seconds. You need to stay right on his tail or super far away to avoid this attack. When you see him leap back, run straight towards him if you have enough stamina and time to get to his tail: chances are he is starting his laser attack. If you're at mid-rage, ROLL AND PRAY.
He will be burnt out after this laser attack so you can go get some free hits in.

Weak to: Thrust, Lightning, Wolf Knight and Farron Greatswords.
Resistant to Magic, Dark and Fire.
Immune to: Poison, Toxic, Bleed, Frostbite.
*You can cheese Midir by casting Pestilent Mist repeatedly. The mist will eat away at his health. So long as you can dodge his attacks and spam it, he will die eventually.

Remember: Target the head.

This boss fight can be easy or hard depending on your opponent, no real tips here apart from killing the NPC in white as quickly as possible. Once you kill the player, the NPC will die off too.

Weak to: Frostbite, Lightning and Fire.
Resistant to: Bleed, Poison and Toxic.

*Counting both Painted Guardian's health and Judicator Argo.

The last boss, congratulations on getting this far! Gael will however not go down without a fight.
The most important tip for this boss fight: If you get 3 swings in with a heavy enough weapon, on New Game you should be able to always stagger him. I haven't tested this, but I know this works for Exile Greatsword, Great Club and every Greathammer. Because of this, do not be afraid to get greedy with Gael if you have those weapons or similar. If you can get 3 swings in in a short enough amount of time, you can stagger him, immediately cancelling any attack he is about to do. My preferred build against Gael is 40 or 50 STR, maybe 25 Faith for Lightning blade (+5 with the priestess ring) with the Exile Greatsword. It hits Gael for about 600 damage per hit, and he gets staggered in 3 hits, turning this boss fight into a cake walk.

He usually swings 3 times at you in his first phase and when you see him do his charge attack where his sword goes red, dodge that and get a couple free hits in. When he does his jump attack where he slams his sword into the ground, it's a bit delayed so practice it. You can get some more hits in when you do that. Using a quick weapon such as straightswords, maybe greatsword is useful against Gael; his attacks can be very sudden.

In Phase 2, he usually does 2 or 3 combo attacks, but in Phase 3 he nearly always does 3 combo attacks. Watch the ground for the lightning as well and be aware that his sword has extended range due to his cape/hood hitting you after the sword.

Weak: Frostbite, Poison, Toxic, Hollowslayer Greatsword in Phase 2 and 3.
Resistant: Bleed, Dark Damage in Phase 2 and 3.

Recommended Weapon: You can use the Irythill Straight sword and in 5 or 6 hits you will cause Frostbite, dealing massive damage as he is very weak to frostbite. You can also use Vordt's hammer or Friede's Scythe, but they are way too slow and he will hit you most of the time. The Irythill straightsword is your best bet here. If you do use these frostbite weapons, you should look up how Frostbite build up works, but basically: If your opponent gets burned by any means, it resets the Frostbite meter so you can constantly frostbite. If you do this, I recommend you use the spell Fire Surge to quickly burn him or if you don't want to invest points for that, you can just simply throw firebombs. You should have enough time to chuck a firebomb at him between his attacks.
*You can also cheese this boss by poisoning him constantly by any means and just running around. He eventually dies off.

*This is his Total HP Value

Congratulations, you won!

I hope this guide helped people out who where struggling with bosses. If you really struggle, don't hesitate to use summons.

Changelog:

• Corrected a few boss weaknesses and resistances
  
• Added health values for NG to NG+7 for every boss
  
• Added that the King of the Storm has an attack that may catch you off guard