Here is the full list of all 14 Undergrave achievements.
Welcome to Undergrave
Congratulations!
Push an enemy into the sword.
Defeat the Beastmaster
Defeat the Big Slime
Defeat the Twin Half Spiders
Defeat an enemy with an explosion
Defeat 2 or more enemies with 1 Dash
Unlocks the second playable character
Unlocks the third playable character
Defeat 100 enemies
Take damage from 2 enemies in the same turn.
Drop a sword on the head of an enemy.
Cheese a boss by dropping a weapon on it's head.