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