This is a defense heavy mission that'll have you protecting your base from an endless onslaught of infested Terrans that only attack by night. By day you'll send out a force to raze all of the Infested Terran buildings.
1. Set up your defense
As usual, get your resource production going immediately and build 3-4 additional SCVs. Upgrade your two barracks with a reactor and a tech lab. Build an Engineering Bay and begin researching the weapons and armor upgrades.
Your main defense points will be to the left and right of the Colony Ship up on the ramp, and the South entrance to your base that's temporarily protected by debris:
Have 2 SCVs build bunkers at these points and immediately pile 4 marines into each. Position the Firebats and Medics you've been given to the left or right of the bunker and have them hold position. Finally, build a supply depot to the left or right of the bunker to seal up any space in your line that enemies could slip through. Leave 1 or 2 SCVs there during the night to repair the bunkers and have them harvest during the day. This defense will easily get you through the first night few nights. Enemies will not assault the base from the south until the second night at earliest so you can put off building there, but make sure you have a defense there lest you get overrun.
2. Watch for aberrations
The only thing to look out for are the "Aberration" infested Terrans, large melee creatures that do massive damage, As soon as you see one approaching focus all your attacks on it as it can quickly ruin your defense.
3. Assault the Infested buildings
After your defenses are set up, begin building a squad to destroy the infested buildings during the day. You can opt to build a large squad of quick marauder units but as long as you plan ahead, you can have a squad of Marines do the destruction and get back before nightfall. You'll be given 3 Hellions soon after the first night, which can be helpful for quick strikes, but actually may work better for defense because of their long range flame throwers. The Infested buildings are defenseless during the day except for some small Zerg broodlings that spawn from wrecked buildings. They do virtually no damage but it can add up over time, so bring a medic with your squad. Start heading back to your base when you get the 30 second warning.
4. (Achievement) Kill the Infestors
You'll be informed of Infestor units that only appear at night but can be killed for additional Zerg research points. The Infestor units' location is displayed clearly on the map, but unless you've already cleared the area of buildings it can be dangerous to approach them at night as infested buildings spawn dozens of enemies at night. Make sure to clear the buildings on the middle right and lower left first to make it easier to kill the Infestors, they're pushovers that can be killed with either your Hellion squad or a few marines.
5. Wrap it up
Destroying buildings decreases the number of Zerg that spawn from that area, so clearing a lot of buildings around one of your defense points will decrease the intensity of the attack as the nights go on. Make sure to shift your mobile forces to the side that has more Infested buildings so you're not caught off guard.
You'll have more than enough resources to handle this mission, just make sure to always be producing units to strengthen your daytime attack while defending. Make sure to monitor the attack points for Aberrations and use your daytime attack crew to help out if it looks like a point is getting overrun.