Zero Hour

This mission is more about outlasting the enemy rather than outright fighting. You'll want to guard both of your entrances by building bunkers. However, you don't want to be completely closed off. Instead of making all bunkers and walling yourself in, build at least one supply depot. The depot can be lowered, allowing your army to advance and scout the area.

You'll want the two barracks combo from the last mission: one with a tech lab for medics, and a second with a reactor for many marines. Load your outer bunkers with the full amount of four marines and leave the medics on the outside. To quickly repair your buildings also leave a single SCV nearby and it will automatically mend any damage.

Several rebels and supplies are scattered around the map. Assemble a small squad of marines and medics to find them. The supplies will help you build more troops, and rebel forces will join your side when rescued. The Zerg will attempt to stop your searches, and while you'll likely loose some units, your rewards will outweigh most loses.

Once you've gathered enough allies, mineral, and gas, return to your base and close off your walls by raising your supply depots. Now it's simply a matter of holding off against the advancing Zerg. Your marines are capable of taking out both ground and air units effectively. If you need a little more anti-air support build a missile turret or two. Otherwise just hold out until time expires to finish the mission.