Feist

This achievement is obtainable at the very end of level 9 "Ruins of the Past," when you have to fight all three beasts to finish the level.

Two of the beasts spawn on the top floor and the other one spawns at the bottom. Try your best to kill at least one on the top before going down, because it makes it much easier so you don't end up fighting all three of them. You want to avoid using dart traps (there are 2 of them set up), which are located all the way to the right of the bottom floor. Try to stay on the right side of the beasts and spam-throw logs so they move towards the left, away from where the traps are.

If you happen to mess up and one of the beasts gets killed by a trap, you can suicide and it will restart you right before the fight. The achievement can still be obtainable and you don't have to restart the entire level.

Hope this helps!