Here is the full list of all 20 Rats, Bats, and Bones achievements.
Drown 20 monsters.
Win a game with 1 crystal health remaining.
Kill 5 monsters with a single bomb.
Earn 5 skulls in endless mode.
Win a game without using barricades.
Kill 5,000 monsters.
Kill a monster right next to your crystal.
Deal 20 or more damage with a single critical hit.
Kill a royal guard without breaking its armor.
Rack up 50 kills with a single trap.
Reach act II.
Reach act III.
Beat the final map on any difficulty.
Kill 20,000 monsters.
Unlock all traps.
Unlock all trap upgrades.
Unlock all player bonuses.
Deal 1000 damage with a single trap.
Win every map on any difficulty.
Win every map on Hard.