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