For this you need at least one guy with you.

Fight off zombies in the main room to at least round 4 (I recommend round 6) and open the upstairs.

Get yourself some good guns like the trench gun and open the help room from upstairs.

Then get the mystery box for good weapons like the flamethrower or raygun.

Then one of you guards the windows and another kills zombies coming from upstairs and from the tunnel.

Just have good guns and someone to play with you and this tactic works for sure.