Things are falling from the sky, and it's up to you to pick them up and bin them, all the while trying to avoid being squashed by anvils. The idea for Cleangame comes from the bonus game from Mick and Mack: Global Gladiators on the Sega Mega Drive.
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)