You have to grab Kerrigan, go to the Comm Ctr, and defend it. She's due South, take her, cloak her, and run east through the enemies. You should be able to get her and Raynor through OK (the rest will probably die, oh well). Kick out your bunker guys and destroy the nearby troops. Now build lots of SCs, keep the bunkers full and maintained.
Build up 3 wraiths (and their cloaking ability), two transports, and send down 4 guys, a SC, and a few vultures. Build two bunkers on the island in the bottom right corner. Then build two missile towers. They can hold off anything that comes near the island. Build up a force on the island - vultures, wraiths, and some regular troops.
Go just across to the other side of the bridge, and build a barracks and missile tower. Keep doing that across to the left, slowly. They've got a pretty good force, but they'll be decimated by the bunker/tower combination. Soon enough you'll eat through their entire base!
StarCraft Walkthough Index