this is how i normally do it with 3 other mates to get to around level 15.
level 1 - knife all zombies
level 2 - use a whole clip of the pistol on the zombie then knife
level 3 - person with most point open door, then everyone buy the MPL off wall. If everyone still has enough open the elevator and go down.
level 4-6 - there are 4 windows on this floor so everyone should be able to take a window. There is an Mp5k on the wall next to the US flag
level 7 - same as 4-6 but at the end of this round keep a crawler, but the thing blocking the stairs, go left buy the elevator, go down, turn power, look for box, and when everyone is done, camp in the elevator.
level 8+ stay camping in elevator 1 person takes the window, 3 takes door. 1 of the person taking door should stay near the switch to go up or down in case of emergancy. end of rounds keep crawlers, get perks, PaP, etc.
Also when its the pentagon thief round, switch to a rubbish gun so he takes it and you can kill him with your better gun.
hope this helps.