This mission will have you escort convoys of refugees through a Zerg infested area. You'll need to get at least 50 of them to the extraction point to complete the mission
1. Push forward
You start the mission with 2 medics and 2 Firebat units. The Firebats use short-range flamethrower attacks that are devastating against melee units like Zerglings. They don’t have much range though, so be careful of Hydralisks. Push forward to the encampment, there are a few Zerglings along the way, but they shouldn’t give you any trouble, feel free to pick up the minerals and gas deposits on the way.
2. Get to the encampment
You'll be given control of the base. Immediately build 3 additional SCVs and start building as many Marines as you can. The barracks you're given has a tech lab attached so you can build additional Firebats or medics, but 2 medics will be plenty for your patrol squad and the existing Firebats will be plenty as well.
3. Accompany the Convoy
Each convoy will need to be defended at 3 points: immediately to the left after leaving the base near the first bunker, on the right near the opening after the first bunker, and at the third bunker on the left. The first convoy will leave soon after you take over the base but don't worry about it, your Firebats and medics will be adequate as long as you stay near the convoy.
4. (Achievement) Collect the first Zerg chrysalis
The first Zerg chrysalis will be directly to the right of the extraction point, you can't miss it.
5. Set up your defenses
Once the first wave is over you should have enough marines to at least partially fill each of the premade bunkers, do so. Continue building marines as you're going to want a small force to accompany your medics and Firebats; they'll be your point team. Once you've got enough marines built for your main squad, send an SVC out to build a bunker at the second Zerg attack point and fill it up with marines. For maximum efficiency, build an SCV for each bunker and have him stand behind it so he'll automatically repair it.
6. (Achievement) Collect the other chrysalises
During the second wave you'll see the second chrysalis to the right of the convoy path near the second opening/Zerg attack point. Go in and grab it, but be careful as you'll be in extremely close proximity to the Zerg base, and it's filled with way more enemies than you can handle.
The third chrysalis is to the left of the first bunker, down the path a little bit. Ideally you'll want to snag this right after pushing back a wave of Zerg at the bunker so you won't run into a large squad waiting to strike.
If you can set up the full bunkers with SCVs repairing them at each of the three points you really shouldn’t have a problem with this mission, just keep your squad close to the truck and make sure none of the bunkers are taking too much damage.