All you have to do is both of you get a ray gun, shotgun, or any other fast gun.
Go upstairs through the help door.(its important you dont go through the main room) After you go upstairs go through the doorway with the window and sit in the corner under the grenades. With 2 or more people this should be easy since all you have to worry about is 1 window and i doorway. Doing this I got to level 22 with a friend.