Note: this will only work if you haven't updated the game on your console.

Unlock the help room once you have enough to randomize a couple times, then camp until you get an automatic adn the M2 flamethrower (but with 2 people have one get any weapon).

Then unlock the upstairs (help room stairs) then in a corner between a pillar and the half broken wall with a slanted board in it (the on with the M2) run and jump towards it and go prone (get on your stomach) in the air and land in the board.

Hit A once then turn around and prone again (then you're in an invisible barrier).

The 2nd peson (with the sniper) should go in the corner across from the window next to the trenchgun and then have him snipe while the other guys owns with the M2.

I got to level 21 with this alone, I only failed because I threw a grenade and it bounced and killed me, so don't use them unless you are the guy in the corner sniping.