Quickest and most efficient method!
Round 1 - 1 shot and knife zombies.
Round 2 - 2 shot and knife zombies, open door for 500 leading into elevator hallway.
Round 3 - 2 shot and knife zombies, open elevator for 750, finish round before taking elevator down.
Round 4 - take elevator down, clear debris for 750, clear lab elevator for 750, clear server room debris for 1000, open server room for 1000, bank 500 for the elevator ride, I would suggest thinning out the zombies with the chain gun before you take the elevator. Save a zombie and take the elevator to the lab. Turn the power on and then go to one of the teleporters and wait for the big desk symbol to take you back to the war room. Once you get back go around and turn on the 4 defcon switches as I show in the video. The server room switch should be the last one you hit do to having limited time for this to work. Once this is done you can go to the war room teleport and it will have a head, take it.
Every time you teleport, back up or turn around back into the teleport. You can’t wait for it to turn blue you need to be in it before it turns. If you do this correctly you will have just enough time to get 7. The reason this works is because the panic room is available for a limited amount of time so you can keep going back and forth. This will save you from having to memorize a pattern or waiting on cool downs by running around the map to different teleporters.
This can be done on casual.