Super Sunny Island

Each level has several enemies. You don't need to kill them, but they are often in the way so it's better to get rid of them as touching them will hurt you. Shoot them with [cn+X] to make them go away.

Kill 50 (cumulative across all levels) for this one. The achievement tracker does not work for this game, but fortunately you only need 100 and should get there way before getting to the level 2 boss.