When you get to level 4 have the person with the highest points open the help room. When in there have someone to protect the two windows and wall.

Have two people stand guard at the door when someone is getting a new gun. Then switch off with each other until you have good guns for all of you.

Then when the last zombie of the round comes make him a crawler with a grenade, then fix all the windows and by the stairs in the help room and go up.

Then go into the corner near the other stairs and shoot the crawler and stay there and cover each other while reloading.

When the last zombie of each round comes make him a crawler and fix all the windows and when there is a nuke or max ammo go get it!