Easy enough - as the achievement description says you just need to bounce on the heads of 100 enemies.
If you're after a quick way to do this rather than natural progression I'd recommend heading into the first level of the game, when you reach the dungeon section you'll be taught to punch by pressing X, here there will be a chasm with ghosts endlessly pouring out - these don't stop spawning so you can just bounce on their heads and repeat until you have the 100.