If you max out Alice Jeezebel (the hero that turns enemy corpses into skeletons and skeleton archers), her Resurrect area is HUGE and can rez lots of enemies at once. If you max out your skeletons and skeleton archers (which is cheap to do), you can have a free, infinite army of powerful skeletons. Not that anyone will see this after the next 5 comments :P jk:P
a timer would make all the difference and if your screen starts shaking or no balls keep on coming keep the last one in the air as long as you can to waste time