Explodemon

Acid will first be introduced in the third world. You simply need to kill 100 enemies by knocking them into the acid that is found throughout the levels. This trophy is first farmable in world 3-1 after being taught about the acid. There will be a spawner next to it and you can stand to the right of it and blast the enemies into it for a few minutes until the trophy pops.

Thanks to legndorock for the tip.