Throughout them game, the player must face many bosses. Below is a list of them, listed in the order they are fought. Some bosses are fought twice.
O'Chunks
O'Chunks is a boss fought in Yold Desert, Gap of Crag, Floro Caverns, and Castle Bleck Entry. The first time fought he has a max HP of 20 and attack 1. The second time fought, he has a max HP of 40 and attack 2. The third time fought, he has a max Hp of 60 and Attack 3. The last time fought, he has a max of 100 and attack 4. O'Chunks will mostly try to jump on you and shoulder ram you. The best strategy for defeating him is catching him with Thoreau, throw him, and jump on him. At the end of the game, he wanders around Flipside.
Fracktail
Fracktail is a giant, mechanical dragon fought in Yold Ruins. At first, he has no intention of hurting Mario and will gladly let him pass. Unfortunately, Dimentio comes and puts a spell on him, making him glitch and turn hostile on Mario. He has an unknown max HP and attack is 1. To defeat him, you must first jump on its back when it comes swooping down. Once you are on its back, in the air, use Thoreau to grab a Frackle, (the robotic creatures that walk on its back) and use it to hit Fracktail's antenna. After a while he will knock you off. Repeat this process until he is defeated.
Mimi
Mimi is a boss fought in Merlee's Basement, Sammer's Early Duels, and Castle Bleck Foyer. The first time fought she has an unknown HP and attack is 1. The second time fought she has a max Hp of 25 and attack is 2. The last time fought, she has an unknown HP and attack is 4. The best strategy to defeat her is to jump her.
Bowser
Bowser is fought in the Bitlands and the Underwhere Road. The first time fought he has a max HP of 20 and attack is 2. The second time, he is much harder with a max HP of 80 and attack is 8. The player can place bombs next to him to take damage. Jumping i on him is useless due to its spikes.
Big Blooper
The Big Blooper is the boss fought in the Tile Pool. As the player progressed through the level, they found Big Blooper's tentacles. Big Blooper will shoot up its tentacles from under. The red tentacle is the weak spot. It can be hit with Bowser's fire breath or with Thudley. Hitting the more common blue tentacles will increase the frequency of the red tentacle appearing. He has an unknown max Hp an attack is 2.
Dimentio
Dimentio is a boss fought on the Dotwood Tree and in Castle Bleck Interior. He is able to flip and manipulate dimensions. The first time fought he has a max HP of 30 and attack is 2. He sends both of them to a dimension he created called Dimension D. He claimed that the dimension made his attacks 256 more potent. However, he overlooked the fact that it also made Mario's attacks 256 more potent. At Castle Bleck he has a max Hp of 80 and attack is 4. The best strategy to defeat him is to attack him the moment he is winding up his attacks.
Francis
Francis is the World 3 main boss. He is also found in Castle Bleck but that battle is optional. He is a geeky chameleon that captures Tippi. He has a max Hp of 40 and attack is 1. He can turn invisible and the ability to send is army of robotic kittens. When he is invisible look at the floor for his shadow then, predict where he'll move.
Mr. L
Mr. L is Luigi brainwashed. He is fought in the Whoa Zone. He has a max HP of 40 and attack is 3. He will jump very high which can become a problem for the player. He will also attempt to heal himself with a Shroom Shake but can be stopped by using Thoreau to grab it from him.
Brobot
Brobot is a giant machine controlled by Mr. L. He is fought in the Whoa Zone after Mr. L is defeated. He is fought in space and the player must use Squirps to blast it. It can shoot missiles and inhale the player. It has a max HP of 255 and attack is 4.
King Croacus IV
King Croacus IV is the world 5 boss. He has 3 smaller heads he uses to attack. He has a max HP of 50 and attack is 2. The player can use Thoreau to grab one of the heads and throw it at the main head.
Brobot L-Type
Brobot L-Type is the improved version of Brobot. He is fought in the World of Nothing. This time he has feet and hands making him far more powerful. He has a max HP of 64 and attack of attack is 5. The best way to defeat him is to use Boomer to blow at its feet.
Underchomp
The Underchomp is a boss fought on the Underwhere Road. He is based on Cereberus, the the three-headed fog that guarded the Underworld in Greek mythology. In battle, it is a parody of Pokémon, music and style. It has a max HP of 16 for each head and attack is 3 each. It is wise to battle him using Bowser and Thudley, dealing quadruple damage.
Bonechill
Bonechill is an undead dragon with a long neck, six small wings, and a golden cannon with two wheels. He is the Chapter 7 boss. He will drop icicles from the ceiling a will shoot icicles from his cannon. He has a max HP of 80 and attack is 4. The player can use Thoreau to grab the icicles and throw them back at Bonechill.
Count Bleck
Count Bleck is a boss that at first, seems like the final boss. Count Bleck is invincible at first invincible with the power of the Chaos Heart. Its power is then negated with the power of the Pure Hearts. He has a max HP of 150 and attack is 8.
Super Dimentio
Super Dimentio is the final boss in Super Paper Mario. It is a fusion of Dimentio, Mr. L, and the Chaos Heart. At first it is invincible, but through the power of the Pure Hearts, it became vulnerable. It has a max HP of 200 and attack is 6. The best way to defeat it is to jump up on the platforms and jump it.
Wracktail
Wracktail is an optional boss. It is found at the end of the Flipside Pit of 100 trials. It is identical to Fracktail except for its color and it uses the same battle strategies as Fracktail. It has an unknown max HP and attack is 10. To defeat him you must use the same strategy as Fracktail.
Shadoo
Shadoo is an optional boss that appears at the end of the Flopside Pit of 100 trials. At first, the heroes do not know that Shadoo is a boss. He asks the heroes to go back and do the Flopside Pit of 100 Trials again. This was only to observe the heroes' moves. When fought it will take turns turning into a dark version of each hero. Each version has a max HP of 100 and attack is 10.
End Boss
The End Boss is the World 6 boss. He is the 100th Sammer Guy. The End Boss is very powerful: He has a spike on his head so jumping on him is useless, he can breathe fire, use a rolling attack, and attack with his spiky club.