Solo i tend to get everything open ASAP then buy jugganog/Speed and camp on the balcony and let them run in straight line towards me, got to round 38 on solo doing this way.
With 3 or more people let them camp on the stairs and you just stand near where the box originally starts, that way you get the majority coming from straight infront and you don't get any attracking you from behind because they go up to the balcony, this one got me and 2 mates to round 33.
Well you could open up the double barreled shot gun room. Then and work your way over to the trench gun area but dont open up the door to the trench gun. one person watches the window and the other people chill and take out the zombies coming straight down the way. Me and some friends got to round 36 just doing that with no glitches or anything. If anyone wants me to show them my Tag is PonyPwnage420