1.On the second thought, endless wave of strong enemies doesn't look as good as I thought. If you want to kill player for sure, then why don't you just implement good old timer and make the game say "you lost" when time is up? (Of course you also would need to show the time left during the game). Besides, some players will be upset/angry when in vain attempt to stop the unstoppable wave they will spend their bonuses, especially expensive ones.
2.Related note: for such missions currently the game says that a player must reach the base of enemy in order to win. But it's only half of truth. Player can win just by destroying all enemies, without reaching the enemy base. Probably you could either add this additional winning condition into description or change code to prevent winning when all enemies are exterminated, but the base is not yet reached.