Here is the full list of all 20 Crystal Story: The Hero and the Evil Witch achievements.
Defeat Rock.
Defeat Bop.
Defeat Rock twice.
Defeat Bop twice.
Complete the missing cat quest.
Complete the thief quest.
Win your first battle.
Win 100 battles.
Complete the bestiary.
Create 10 items using Alchemy.
Create 25 items using Reforge.
Open 30 treasure chests.
Reach level 30.
Defeat Fury.
Defeat Bunch.
Defeat Bahumot.