Better solution and a feature we can use in all other modes...
Allow us to speed up creeps spawns during waves, like most other tower defense games have. Got your defenses set up? Hit a key and the next set comes out immediately. Option gets disabled when another player joins. Thus, if you've got all your towers fully upgraded, just spam the key and let the mobs die, en masse.
Ever notice there are five "time positions" on the wave progress bar? Start, first quarter, second quarter, third quarter, and finish... Each time you hit the key, it pushes the progress bar to the next mark (en