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