We did zombies one but it was like 8 on 8. Humans could'nt leave a certain maybe 20X20 foot zone and the zombies could go wherever they wanted. You get shot, your a zombie, go to zombie spawn and re-insert. It got pretty intense but we humans held out with 2 or 3 left. Good times.
Or just do gentlemans war (could be the same as Murph). Say its 4 on 4. Find an open. flat area. Stand in a line next your team mates with the opposition, say, 40 feet away. Flip a coin, winning team shoots first. Each player gets one shot. No turning, ducking, running or really any moving in any way. Just protect your nuts. Continue until one team wins. Gun hits count only because you'll likeley be using it to protect your junk.