Beat alien,king,pirate, and gemma campaigns, has 12 red bricks and 999 studs. and unlocked all alien characters. plz pardon my spelling the keys on my coputer are kina broken.
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)