Clients, menus, firmwares and all other flashcart related files for GameBoy Advance. Remember to also check the DS Flashcart Files section as many GBA capable carts were released with the DS in mind.
I don't have a wii u anymore to test it myself, but if homebrews are not visible on the wii u menu I think you can press L + R + minus to open the plugin menu, there should be an option called "homebrews on wii u menu" or something similar
Or when your player dies, you can say in code, if enemy exists, do nothing, but if enemy does not exist, then create enemy at certain spot. (This would be a pain tho for lots of emeies)