To earn this achievement that's called "Bone To Pick", you'll need to hit the boss Skullmageddon with a hair pick.
This is done when you enter the Rocket Ship during Stage 2, which is called Back Alleys. Once you regain control of Billy or Jimmy, keep pressing right until you see a bat and a trash can. Instead of taking the bat, destroy the trashcan and pick up the pink hair pick, then continue on and confront Skullmageddon for the first time. Once the fight begin, just get close enough to him and hit him once with the hair pick. If you do it succesfully, the achievement should unlock.
Have fun and good luck!