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
Free Lil' Willis 10 times
Complete The Deadlands
Complete all of Poe's requests
Complete Apricot Acres
Collect 150 Stars
Open 25 treasure chests
Complete Nogport Meadows
Collect 5 costumes
Get 5 'Spare all Boogins' challenges
Complete Powder Peaks
Use Poot to toot on a Boogin
Freeze a Boogin into a block
Eat 100 pieces of cake