Simply buy any of the following: Storm puff / Mega / Super Mega.
Ideally the last one. They can be bought from the Items Shop in town.
Easiest place I found to do this quickly was Mermaid Falls, the crabs only take one hit. Just equip it as your power (LB/RB) in the top left and then press B while facing the enemy.
Do it 20 times and the Achievement is yours.