This strategy works every time and has gotten me to my highest level.
First things first, you have to start off on the correct side if you're with two people, or if you're with 4 you have to both understand the strategy. But lets just do this for 2 people cause the 4 people variation is just connect the dots after you read this.
If you start on the Jug side, immediately start over and keep starting over until you get to the right side. Once you're there kill zombies until round 3 or 4 and open the door and buy the Thompson. once you do that hangout in there until you have enough points to link all the doors until power. turn on the power leaving that back door closed. (you may open if when the box moves if it is behind there) When you're here if you have enough you can hit the box at will and get your perks ect.
Now it's time to go back to the jug side. open those stairs and have one person control the front by double tap and the other control the window behind by the trap. There is only two ways they can come in this scenario. Do NOT open that last door. I repeat Do NOT open that last door! (PS you can get your perks anytime you want at this point if you haven't already)
Buy your bouncing bettys and place them strategically. some in the back by the door you're not going to open, and all along that strip where the back window is just in case.
You can stay in this room each person alternating between taking the front and the window, because the front gets more points and you could base it off who has the best gun ect. If you get overrun with zombies just run over to the trap side by the window turn on the trap and it will kill everything besides the ones coming out of the window. Rinse and repeat
you can stay here doing this strategy until round 19-22. at that point you're going to want to open that last door. Once you do Keep the stream of zombies following you. AT MOST only kill have the train. This controls the spawning. None of them will spawn ahead of you because they are all chasing behind you. Rise and repeat until you die.