Here is the full list of all 20 They Are Beasts achievements.
Kill one enemy
Kill 1000 enemy
Kill 5000 enemy
Kill 500000 enemy
Kill 50000000 enemy
Kill 1,000,000,000 enemy
Die once
Die 13 times
Kill Haron
Kill main boss!
Kill main boss 13 times
Complete the game
Complete the game 5 times
Solve the whirlpool puzzle
Complete the cemetery level
Complete level 1 of the dungeon.
Get 50,000 points
Get 100,000 points
Get 10,000,000 points
Get 1,000,000,000 points