Me and my friend got to 20 with the help of rayguns, trench guns, and LMGs

I got caught in a corner, however, and promptly died on wave 19. My friend did surprisingly well without me, though. Though I respawned on wave 20, I did terrible once I got back. I had plenty of money for a trench gun off the wall and possibly an LMG from the random box, but I could get neither in time; I died almost immediately.
My friend lasted about 30 seconds on wave 20 without me.
On my own, I made it to wave 10. The damned zombies pulled a ninja sneak attack from behind.