Here is the full list of all 15 Hack, Slash & Backstab achievements.
Kill 1000 Enemies In One Round on 'Mean and Cruel' or 'Brutal Insanity' difficulties, then complete the round
Search Every Room in a Dungeon
Defeat The Skeleton King on 'Mean and Cruel' or 'Brutal Insanity' difficulties
One player wins the game on 'Brutal Insanity' difficulty with no backstabs
One player wins three rounds
Defeated the Spider Queen on 'Mean and Cruel' or 'Brutal Insanity' difficulties
Defeated the Dark Wizard on 'Mean and Cruel' or 'Brutal Insanity' difficulties
Defeated the Dark Rogue on 'Mean and Cruel' or 'Brutal Insanity' difficulties
One player uses a phoenix feather to revive three players at once
Defeat the Dark Archer on 'Mean and Cruel' or 'Brutal Insanity' difficulties
Defeated the Dark Warrior on 'Mean and Cruel' or 'Brutal Insanity' difficulties
One player wins the game after backstabbing all other players on 'Mean and Cruel' or 'Brutal Insanity' difficulties
Win a game on 'Brutal Insanity' difficulty with no friends