StarCraft Walkthrough: 3: Desperate Alliance
You've been abandoned to your fate. Boo hoo. You have to survive for 30 minutes. It's really not that bad - build 2 bunkers and 4 missile turrets at each entrance to your area. Build a few more missile turrets amongst your buildings to get flying things. Then just hunker down, enhance everything, lay spider mines to watch their approach. The last assault is of course with only 10 minutes left, just ride it out.
StarCraft Walkthough Index