Ha, that's very clever! You can definitely push things with the permanent upgrade minions and free zombies. You do have to be careful though, as each wave gets harder so you could potentially get yourself into a bad situation if you delay too long. I really don't mind "broken" combos in the game (they can be fun), but anything that seems like too much of a "grind this optimal strategy every game" is a negative gameplay effect I'd like to address so I'll review this one.