Here is the full list of all 18 Bat Boy achievements.
Defeat Azaros.
Defeat Azaros for the second time.
Defeat Azaros for the third time.
Buy an item.
Play a song in the Groovy House.
Collect all Cassettes.
Buy a Golden Seed.
Defeat Vicious Maximus.
Collect every single seed.
Collect 999 Crystals.
This room looks familar... It needs more ghosts...
All your friends are in the Groovy House.
Deflect 100 projectiles.
Score 5 Homeruns.
Give Boarman an ingredient to prepare a drink.
Give Boarman all three ingredients to prepare drinks.
Find and rescue a pet.
Find and rescue all pets.