First you should get 4 equally good players preferably with mics. Have a player lay prone in a corner and have another jump on top of him (this will give you more drops like ammo and insta kill). If done correctly, you will hear a little girl laughing. Revive the downed players and get to your windows. When you start out the zombies only come out of 4 windows. 4 people 4 windows (pretty simple) stay at your windows as long as possible (I usually stay till round seven or so depending if i buy the rifle or not. Best technique is to fire a few rounds into the zombies and then knife them for the most points per zombie. In round one I think it is 5 or 6 shots with the pistol to get a kill (this will increase every round). Bullet spray kill is worth 10pts per bullet and 50pts at death, head shot is 100pts, and a knife kill is 130pts. So buying the rifle off the wall is a decent investment since the cost is only 500pts (only equivalent to 4 or so zombies) and this will allow you to stay in the room longer. Now this is a slightly more experienced tactic, but when you feel the round is nearing and end you can do, what I call, a "controlled breach". This is allowing your zombies to come through the window in hopes of receiving some sort of drop. Drops should be used in the following ways: insta kills, double pts, and death machines should only be picked up when round has started and zombies are already at windows or drop has gone through its second stage of blinking, but if your team is overwhelmed, by all means take it. Bombs will lose you points if not done correctly. They give every player 400pts, but if you have 3 zombies at your window and someone picks up the bomb, you just wasted a few hundred points. So what you want to try to do is wait till in between rounds, this will ensure you have received all the kill pts you can and you will get the 400 from the bomb (800 if its double pts). Now with every one in the team ready, open the door and head straight down the elevator. I think it's useless to stick around and try to cover all the windows from the elevator. now when you get downstairs (DON'T BUY THE DEBRIS) just have everyone take a window like before (4 people 4 windows). I prefer to have two sub machine guns in my arsenal, this will come in handy if someone goes down or you get overwhelmed. Communication is key! if you are low on ammo SAY SOMETHIN! so you can have you team watch your back while you resupply. Just stay at your windows until everyone is ready and has sufficient pts. I usually stay till 14 or so and then I have 20k+ (more than enough for perks claymores and 2 good guns from the box maybe monkeys and pack a punch if needed. Before you open the debris, make sure to leave at least two crawlers. Best way is to have everyone try just to be sure.

I'm not saying this is the absolute best strategy out there, but could help newer players gain some exp. Ill add a more indepth strategy for later rounds at another time.
also my psn is kickbabies if your down to play!