This is the first mission where the enemy actually attacks you deliberately, and you will have a chance to conduct a large-scale attack. 
You start out in a well-protected base with 2 SCVs, some stationary Marines, and 2 bunkers, one at each exit of your base. The bunker at your base's lower side is on fire, and the Vulture next to it is also damaged. Quickly repair them with a SCV before the Zerg attack again. While you are repairing, move the Marines on the platforms down as they might be overwhelmed by attacking Zerg. As soon as possible place 4 Marines inside each bunker and make sure the bunkers are always at full health. Then, begin your resource-mining operation.
As one of your SCVs are repairing, send the other one to gather minerals and begin to build more SCVs. At the height of your operation you should have 6-8 SCVs gathering minerals. Make sure you have 2 to build structures and at least one SCV at each base entrance to repair the Bunkers and Vultures. When you have this accomplished, build an additional 5 SCVs to gather Vespene Gas, using the first one to build a Refinery. Remember to keep your Supply limit above your current supply to speed things up for your attack.
When you have Vespene, upgrade your Infantry Weaponry and then Armor, and also the U-238 Shells and Stim Pack. 
As you are upgrading, begin building a large force of at least 36 Marines and 12 Firebats. Your attack force should be ready when there are 10 minutes on the timer if you have not been devastated from Zerg attacks due to lack of repairs. Move your forces northeast into the large Zerg base. Make sure your forces are together and that your Firebats do not wander off. Especially avoid using Firebats against Sunken Colonies. If you manage your forces well, you can take out almost all the Zerg on the map before the end of the misson.
An alternate strategy is available for people who do not like attacking. You can simply build 3-6 Bunkers at each entrance, with 3 Marines and 1 Firebat in each, and wait. Be sure to line your base with Missile Turrets as Zerg Mutalisks will fly in above your SCVs and kill them. When there are less than 3 minutes, all the Zerg will attack your base simultaneously at the same entrance. If your Bunkers are destroyed, lift your buildings to the safety of your other Bunkers and wait until the mission ends. It's easy, but a bit boring.
