Mission: Zero Hour
This is one of the tougher early missions. You simply need to survive. Your objects are:
- Main: Hold out for Evacuation
- Bonus: Rescue Rebels
Once the mission begins you should immediately save for a Refinery and build a number of SCVs. You're going to need a good flow or resources so that you can build new units. You'll already have some Marines and bunkers at the beginning of the mission - stick the Marines and bunkers and build more Marines/Medics when possible.
You'll need to set up base defenses throughout the mission. I highly suggest grouping your bunkers in pairs. Place a pair at each bridge and a pair at the top of the ramp. Some of these bunkers are already built, but if they are destroyed, rebuild them ASAP. Speaking of which, you should have one SCV stationed at each pair of bunkers. These SCVs can repair the bunkers, greatly increasing their lifespan.
There are three groups of rebels that need to be saved throughout the mission. The first appears in the northeast side of the map. This small group of rebels is fairly close, so you can rescue them with a force of about eight or ten Marines. Once you've rescued them, head straight back to base.
You'll have to endure numerous rounds of Zerg in the meantime. Remember, keep your SCVs around to repair the bunkers. Also, make sure you're constantly pumping out units. Build more Barracks as necessary - I suggest three in total.
A second group of rebels will contact you. This group is in the north. Take a group of about twenty Marine/Medics and head in their direction. More Zerg will be in your way than last time, so proceed carefully. The Medics should keep your Marines from dying from attrition. You'll have to manage attacks against your base during this time as well, so make sure a reserve force is kept at base.
The final group of rebels will ask for your help. They are in the far north-northwest. You're going to need a fairly large force, the same size as last time or larger, to reach them. You also still need reserves. In this case, however, you want to act quickly - you're coming towards the end of the mission. Grab the rebels and head back to base ASAP.
As the mission comes towards the end a huge group of Zerg will begin to advance. If you want to hold them back as much as possible, I suggest building more bunkers and using about half of your SCVs to help repair them. Still, this will only matter for a short time because Zerg spores will eventually begin to land in your base. Do the best you can and wait for the timer to run out.