Here is the full list of all 55 Muffin Knight achievements.
Collect 50 muffins in one run in the Castle level
Collect 100 muffins in a CO-OP match
Kill 200 flying monsters
Activate Prince mode
Defeat the evil lava king
Kill a total of 3000 monsters
Kill 100 enemies with the Knight
Kill 500 large monsters
Collect 50 muffins in one run in the Windmill level
Kill 4 enemies with the Frog tongue
Collect 50 muffins in one run in the Ghost Town level
Get the first muffin in an Online Versus match
Collect 10 muffins without killing any enemies
Collect a total of 1000 muffins
Kill a total of 2000 monsters
Kill 1000 small monsters
Drop a poop on an enemy head
Kill 100 enemies with the Mage
Kill 100 enemies with the Gnome
Kill 100 enemies with the Archer
Kill 100 enemies with the Cyclops
Kill 100 enemies with the Ninja Kitty
Kill 100 enemies with the Dragon
Kill 100 enemies with the Monkey
Kill 100 enemies with the Unicorn
Kill 100 enemies with the Zombie
Kill 100 enemies with the Grizzly
Kill 100 enemies with the Bird
Kill 100 enemies with the Frog
Kill 100 enemies with the Mr. Rainbow
Kill 100 enemies with the Bull
Hop on 100 enemies using the Monster Hopper Perk
Suck 5 enemies with one Black Hole
Kill 5 enemies using a single Unicorn poop
Kill 10 enemies with a Mr. Rainbow explosion
Collect 50 muffins in one run in the Forest level
Collect a total of 3000 muffins
Collect a total of 5000 muffins
Kill a total of 10000 monsters
Win in 5 consecutive Online Versus matches
Win a total of 100 Versus matches
Collect a total of 3000 muffins in CO-OP matches
Play 300 Online matches
Upgrade all characters to the max level
Reach Character Level 40
Die 100 times by falling in fire
Revive the other player 100 times in CO-OP matches
Do 500 Double Jumps
Hit 6 enemies with one PumpKing strike
Kill 100 enemies with the PumpKing
Collect 50 muffins in one run in the Graveyard level
Kill 100 Enemies with the Sumo Panda
Kill 100 enemies with the Yeti
Deliver 50 Gifts in a row in SantaVille
Reach 1000m in the Beanstalk level