works best with 3-4 people at most, in the room with the double doors and the entrance to upstairs, stand in that left corner all the way in the back of the room. After a while, one or two zombies will run at you. use your grenades for multiple kill streaks/combos. Best to have multiple grenades, so as to aid you in destroying the swarming zombies inside the cabin.
You must stand absolutely still while doing this. Don't touch your mouse or switch weapons in any way, or they will notice you.
m9, m9he, and m9 frag pack is preferred.
I'll post screenshots when I have time.
In the meanwhile, post your results.