Unfortunately the classic map doesn't have a lot to it, so you're reliant on the mystery box, although the trench gun does the trick until later rounds.
I've seen someone on youtube stand near the back window in the help room then when he has a good few following run to the left of the pillars, then back the other side of the pillars to the window again, effectively creating a circuit, and that seemed to work, but I'm too clumsy to pull that one off. http://www.youtube.com/watch?v=yWbBjDCZLdw
I personally go in for the lols and don't have much strategy at all. (No video necessary)
Again, another youtuber Goes to the help room via the upstairs not opening the door direct into there, and one holds them off on the stairs, and the other watches the 3 windows, which is easily possible with decent guns, but it gets trickier at later stages. http://www.youtube.com/watch?v=U-sndNsSU2k&feature=plcp
A friend swears by holding the part upstairs next to the grenades and hoping the mystery box plays ball. I think this works better with 4 people though.
I prefer being single player as well, other people just get in the way haha
