This works both in co-op and solo but you have to do it right.

If you don't then the zombies will break through the barrier within seconds.

First off your are in the main room - get as much points as you possibly can for the first 5 waves of zombies coming at you.

Have someone who has the most points open the help door and you open the help box as many times as you can until you get the ray gun and the flamethrower.

So now go to the farthest side of the help room were the light is on.

Stand by and run jump/crouch in between the two boxes in the corner and make sure you are farther in the bigger barrier.