With some creativity, you can do a lot with enemies that just look different though. The game seems to use sprite collisions for hurting enemies, so you can delete weapon sprites and they'll become unusable as I believe was discussed in another thread. Also, I've played around before with clearing the "idle" sprite of enemies like fish that will charge at you. This makes it impossible to snipe them at a distance, and your only chance to kill them occurs when they're already attacking you. You could probably also edit the monkey or dragon enemies to create a banana or flame turret that couldn't be destroyed without TNT.