The earlier rounds are just too easy. Basically get yourself an automatic gun -- RPK, HK, any sub -- and just earn points. Open most doors up (everything besides the doors downstairs without the box) and then start to save for perks. After that PAP and then you are right.
What i like to do is laps in the middle level. Open up the side barriers (the ones which look like a cleaning blockade) and basically run laps all around the computers and teleporters. You can do this with a friend, although it is a bit risky. AS for the other guys, they should stay in the small elevator with guns like the upgraded HK or RPK, raygun or winter's howl. You should have the same too.
Basically as you dodge zombies downstairs, let them build. You'll run around say about 5 times, then you will have every zombie on you. Every now and then, turn around and just pump them with your submachine gun or LMG. Heaps of easy points, and the bullets go through several zombies. Multi-kills galore.
If the guys in the elevator are in trouble, they can just hit the switch and come downstairs for a break, reloading or helping you out do laps. If they get downed however, it is very hard to go get them, unless you have monkeys.
Oh yeah, make sure that you buy juggernaut first, then probably quick reload. Juggy is essential -- after that if you want to PAP, go ahead, its up to you.