I disagree about making a crawler. Just estimate when there's only 3-5 zombies and hold in an open spot (theatre is great for this) and whittle it down to 1 zombie. DO NOT make him a crawler as crawlers bleed out over time in this game a lot more often than they did in WaW.

This might not be the case and me and my friend ran really bad over the few times we tried to make one, but its just easier to do it this way anyways as you can walk (not sprint) faster than any zombie can run.