What I do to get to a high round is Open the right hand side staircase, dont EVER open the one on the left. Camp at the one on the left, (2 players makes it easier) P1 - Take left hand side zombies P2 - Take right hand side zombies.
If you play solo get the upgraded Ray Gun Mk3, Simulataneously Fire black holes on each side and youre sorted (Or use your regular weapons until things get heated, whip out the MK3 and youll smash it easy) Can get to a very high round this way, with the MK3 they really just cannot come close, and another added bonus, the Valkyrie Drones cannot come down there so youre safe from them.