Honestly, this game is all upon luck, but also depends on where you stand and who is your teammate. The highest level i got to was 22 and that was with me and a friend. We started on the right side, by the revive perk. that is probably the best side to start on bcuz then the zombies dont come up the stairs in the electric room and they have to come through the kitchen n that little door just past speed cola perk. We got lucky and got the ray guns at levels 13 and 15 however and only had the MG42 n Browning until then. also the only perks we had was speed cola and he had juggernaut.
A good way with 3-4 ppl is standing on the balcony, having 1-2 ppl cover the door, one cover the windows looking at speed cola, picking them off (flamethrower would be best there) and have another overlooking the ground level outside (MG, Browning, BAR, PTRS would be best). ALso during the ends of each level be sure to GERNADE one of the zombies so they crawl, then rebuild everything, buy all the perks and weapons you need, return to your locations on the balcony and continue to bust some zombie ass!
with 4 ppl i've gotten to 17 with no ray gun. quick revive is a good perk for it turns into a mess when a member dies, just have someone cover you and be sure to have good communication and a good game plan like this.
hope this all helps.

^