Put 2 bunkers at each of your base entrance, surround the bunkers with supply depot. That will leave a very narrow path in and out. After you save the second batch of survivors with full team, go to the top Zerg base and destroy 2 hives. After that go back base. Gather 2 full army, leave some to defend right entrance, go out save third group of survivors and destroy the left zerg base. Remember to constantly set your barrack rally point to this place so that you can take it down. You must take this down before 1.30 minutes left in the countdown before mass Zerg appears