*SPOILER*
Trick is to run to the south gate, right away, and with rocket launchers kill the first wave. The ghouls stationed there will engage the Super Mutants when they arrive. Then go to the east gate and do the same with the first wave. The north gate is ok, just place a guy there and it should suffice. Then run back to the south gate and start clearing in a wide circle towards the east gate.
You can place a sniper guy on the roof of the building behind the ghouls at the east gate to hold off any stray Super Mutant melee guys trying to make a run for it. Dont bother with mines...ghouls tend to get blown up. If you use mines in other parts of the map then dont forget to get the ones at the north gate....couple of them there.