Shotgun is your friend.
The survivors aren't what did it for me. They barely fired. It was the moment I got the Sawed-off Shotgun. Held little ammo sure, but that just meant it didn't take long fully reloading. I bet it wouldn't be as easy without it. With how many zombies I shot down, none of the survivors were getting any action. I almost felt like trying to lose them all to see if I could fend it off on my own with just that one weapon. Anyways, it was a fun game good work.