Note that the condition says a single wave, not a single full level. What I did was start the very first level on hard and put some reinforcements out towards the incoming baddies. The reinforcements took care of the baddies, there are only about six in that very first wave, no baddies even got in range of the archers. Boom, trophy.
Btw, I did spend a star getting the first upgrade for the reinforcements in the armory. I didn't try this strategy without it, so I'm not sure if it will work without the upgrade.