This works better in solo, but if you're good it will work in coop too.

First after you go into the help room get the ray gun (you must have this) and a .30 cal or MG42. Then when you get the money go up the stairs in the help room and stand right in the corner by the grenades.

Use your MG42 or .30 cal to mow down zombies until you can't reload quick enough. At about level 10 to 12 switch to your raygun (should have full or close to full ammo - about 160 rounds). Use the raygun from now on and make sure to reload every chance you get - you're going to need every round!

This worked for me and I got to level 22 but ran out of ammo for the raygun and the .30 cal.