Here is the full list of all 16 Monsters Ate My Birthday Cake achievements.
Free Lil' Willis 10 times
Complete Apricot Acres
Complete Powder Peaks
Find All Message Bottles
End the Boogin King's reign
Find all the monster companions
Complete The Deadlands
Complete all of Poe's requests
Collect 150 Stars
Use Poot to toot on a Boogin
Open 25 treasure chests
Complete Nogport Meadows
Get 5 'Spare all Boogins' challenges
Collect 5 costumes
Freeze a Boogin into a block
Eat 100 pieces of cake