It's all about co-op and team work. We got a zombie on our side, who stood still blocking the others from us, and we shot from under his legs. (In the crevice of one area)
Or, we would run around and last for a few.
One time we won, we had 4 guys, we ran em up the boxes in the Medical room to the Med Office. 2 infront of vents, one with a machine gun, one with a Grenade/law.
See a zombie, grenade down the vent shaft, machine gun holds em there, and you can law them. The other two had mg, alternated there shots and watched the windows.
If your with a team mate ALTERNATE YOUR SHOTS. When your shooting, make sure he is not, or reloading. If your both shooting, when you both end, the zombie will rush you and get you. This eliminates reloading at the same time of a partner.