There are two ways to get this.
One is to get to Fish World (the world right after the first boss) and find some clouds that are spitting out enemies and jump on them continuously until you get the achievement.
The other way is done in the Cave World. In the third room, if you clear out all of the enemies, another snake will appear to take you to a different room four. Go in that one instead and there will be tons of enemies all floating in the air. Just jump on them until you hit twenty.