There are a total of 10 bosses spread across the 5 worlds. In case you need a refresher or just 1 specific boss, below is a video for all bosses and the timestamps

00:00 Chainsaw Megamaniac

00:38 Big Bat

01:11 Green Hydra

01:56 Red Devil

02:34 Eyeskuller

03:38 Necromancer

04:37 Manleech

05:12 Frog King

05:50 Blue Devil

06:24 Necroking

NAME: Chainsaw Megamaniac
HP: 8
DIFFICULTY: Easy

ATTACKS:
1: He will walk across the room and if you attack him he will rage run
2: When he meets the end of the room, he will stop and let rip his chainsaw across the floor across the room and back and continue Attack 1 until he hits another wall

STRATEGY: Wait for him to reach the end of the room and when he lets his chainsaw loose, unleash as much as you can and then repeat the cycle
NOTES:
-He has an achievement called 'Chainsaw Dance'

NAME: Big Bat
HP: 8
DIFFICULTY: Easy

ATTACKS:
1: Big Bat will bob up and down and shoot out 3 bats in a top diagonal, middle and bottom diagonal pattern
2: After bobbing up and down 3 times, he will sweep across the floor (You can duck to avoid this) and repeat Attack 1

STRATEGY: Constantly attack. If you're fast enough, you can kill him before he uses Attack 2
NOTES:
-Bats sometime drop Hearts when killed

NAME: Green Hydra
HP: 10
DIFFICULTY: Easy

ATTACKS:
1: Green Hydra will walk from the top floor right to left, middle floor left to right, and bottom floor right before repeating the cycle.
2: When at 5 HP, He will lose a head and walk faster. At 1 HP he will be headless and walk even faster.

STRATEGY: Only attack from behind, you can get a lot more hits in without putting yourself in danger
NOTES:
After clearing the first floor, Floating Ghoul Heads will start bobbing across the floors infinitely

NAME: Red Devil
HP: 8
DIFFICULTY: Easy

ATTACKS:
1: Red Devil will spawn at either end and left a wave of fire across the screen
2: After Attack 1, Red Devil will walk to the centre and then teleport, shooting 5 fire balls in different directions before repeating Attack 1

STRATEGY: If you get close enough, you can duck under his Fire Wave and attack him before retreating and repeating the process
NOTES:
-You can attack him quickly after he teleports

NAME: Eyeskuller
HP: 8
DIFFICULTY: Hard

ATTACKS:
1: The Eyeskuller will circle around the stage and shoot out Eyeballs to chase the Player
2: The Eyeballs will orbit the Player and then explode after a while if avoided long enough

STRATERGY: I got none. Just keep moving and don't ever let the Eyeballs trap you
NOTES: None

NAME: Necromancer
HP: 14
DIFFICULTY: Hard
ATTACKS:

1: The Necromancer will appear on the outer platform and shoot 5 Flaming Skulls
2: The Necromancer will then teleport to the middle and summon 5 Giant Flaming Skulls that when they hit the floor will leave a flame for a couple of seconds

STRATEGY: During Attack 1, if you duck, the Flaming Skulls will go right over you. During Attack 2, you can stand near the edge and avoid the Giant Flaming Skulls.
NOTES: None

NAME: Manleech
HP: 10
DIFFICULTY: HARD
ATTACKS:
1: Manleech will walk across the scream and will only mad rush if the Player fires anywhere near him
2: When he reaches the end of the room, he will fire 3 leeches at the other end of the room who will automatically come out of their shell and attack the player before repeating Attack 1

STRATEGY: Don't fire in front of him. If you do, he will charge and you will most likely take a hit
NOTES:
-The number of leeches is infinite
-He has an achievement called 'Ozze It or Lose It'

NAME: Frog King
HP: 10
DIFFICULTY: Medium

ATTACKS:
1: The Frog King will throw 2 Toddler Frogs down before crashing down and shooting 5 fireballs. Frog King will then jump back up and repeat the process

2: At 5 HP, Frog King will stick his tongue out and summon a third Toddler Frog

STRATEGY: Don't worry about the Toddler Frogs too much. Focus on hitting the Frog King when he lands and when he jumps in the air
NOTES:
-The previous Toddler Frogs will die when King Frog jumps back down

NAME: Blue Devil
HP: 10
DIFFICULTY: Easy

ATTACKS:
1: If you are far away, the Blue Devil will let loose a bunch of butterflies after you
2: If near or on the Spike Blocks, the Blue Devil will jump. If he catches you, he will toss you against a wall of spikes

STRATEGY: Jump in front of him so that he will hop over to the other side and keep attacking him when he lands. Rinse and repeat
NOTES:
-Butterflies will sometimes drop Hearts when killed
-He has an achievement call 'Bad Touches'

NAME: Necroking
HP: 18
DIFFICULTY: Hard

ATTACKS:
1: Necroking will sway (Right first, then left) to either platform and summon 5 Flaming Skulls at the Player.

2: Necroking will then go to the middle and left off 8 Green Sparks in all directions before repeating Attack 1

3: When Necroking is at 9 HP, he will then fire a Big Flaming Skull at the Player along with the 5 regular Flaming Skulls.

STRATEGY: When he fire, whatever position you were last in the Skulls will follow. So jump in the air then quickly hop down to avoid the Skulls
NOTES: None

That is all. Hopefully this will make your next run (Or 1 HP Mode run) easier now that you know what boss does what and how to defeat them effectively.

-FingerRocks