Ok so i haven't seen this on here yet, but here it is...
Ok so when you get out of the elevator in the laboratory there is a window on your left.
What you want to do is break the whole bottom half of it(like the table glitch) then jump up on the boards until you get to the top of the door(you have to jump crouch)and walk all the way to the left towards the end of the wall.
I recommend one person up there.
Then the 3 other people will stand in the elevator and one guy watches the window.
All the zombies will go to the guy in the glitch.
Enjoy and i made it to round 23 with this strategy.

P.S. me and my friends found this yesterday.