At one point (many years ago) on the 360 myself and 3 friends got to I think it was lvl 28 or something on Untoten. We did so purely by unlocking only the random box room, then had 1 bloke doing the windows/wall, 2 on the door from the first area, and the final one going between helping the windows/wall guy and the 2 main door guys (by later levels they're literally pouring in non-stop). It only stopped working as one of the players decided it would be a good idea to open up another area which screwed it all up and we died on that same level.
What's the best tactic anyone knows of? What's the max level anyone's got to on vanilla?