Maybe you're using units that aren't meant to deal damage? ;)
You're a bastard, I've studied how the difficulty increases the more you level up your minions. It doesn't matter which minions you use, there's either a gimmicky trick to defeat them or there's no way to defeat them at all!
EDIT: I didn't know you could increase the number of minions you can spawn. I won it instantly the moment i found that button. :P my wrong