My and a friend were playing NZ today and figured out a good strategy:
When you start, hold them off in the first room until around level 4-5 (or as long as you can hold out), there need to be two players (atleast) with ~2000 points. Have one 2000+ player open the staircase in that room, and everyone run to the other staircase and the other 2000+ player open the staircase into the Help room.
We made it to level 16 (with only 2 people in splitscreen), before I accdentally lost the ray gun
With 2 people, Have one gaurding the wall and the stairs (perferably with a LMG, Flamethrower or RayGun), and the other protecting the windows.
3 People: Have one at the stairs, one at the wall and one watching the two windows (considering when you get farther, not much action goes on at the windows)
And 4 people: Have one at each point (stairs, windows, wall).
Just remember NOT to open the door, and when you get a chance (unless you have a raygun and flamethrower/browning) use the "???" box and attempt to get a better weapon, and remember taking a weapon offered to you is optional.
My friend and I also noticed that the building you fight zombies in is the building you go into on "Hard Landing" right after using the M1+Grenade launcher.
From,
Chris.