Hold the Line: Complete the "Zero Hour" mission on Normal difficulty without losing or salvaging a structure.
Zero Hour: Complete all mission objectives in the "Zero Hour" mission.
The Best Defense...: Destroy 4 Zerg Hatcheries in the "Zero Hour" mission on Hard difficulty.
This is a easy mission to beat. Unfortunately, the achievements not so much. For the sake of trying to complete everything, I tried to go for it all. Then I gave up on Hold the Line. And then The Best Defense too. Maybe I haven't gotten a good rhythm down. I'll try mission again, and if I can get the achievements, I'll post my results.
When you start, send your SCVs to mine minerals and queue 2 SCVs at your command center and rally them to your minerals. Start building a tech lab at one of your barracks. Train a marine at the other.
You are given two bunkers on your base platform, and another guarding each entrance further out. Part of the reason Hold the Line is so difficult is that you have to work with such terrible initial placement. I would NEVER have built there.
When your tech lab finishes, you want to train a medic and another marine from your 2 barracks. In the meantime, you should also be training additional SCVs, and making supply depots. Gas is important, but not as important as getting your mineral line set up. You can be lazy with the gas.
After you defend against the initial waves, send your free forces towards the right. If you haven't been already, you will be alerted to some troops calling for help. Rescue them and collect the resources. Beware the zerg units that are burrowed. There are two groups of zerglings and and a single hydralisk waiting. If you are lazy, the hydralisks will eat at least one of your marines.
Queue 2 more medics, and start building a refinery if you haven't done so. If you have the resources, UPGRADE! Do both. +1 dmg and +1 armor is huge when playing brutal. All the zerg units are upgraded, and so should yours.
And, you should be off to a good start. rescue the other rebels when you are alerted to their presence. If you go to their locations early, only the resource pallets will be there. After having rescued the 2nd group, I decided to try assaulting one of the Zerg bases.
Having failed at my assault and leaving my base ill-defended, I ended up losing a turret and a bunker or two. That should give you an idea of how easy it is to fail at two of the achievements. Save the last group of marines on the left side of the map if able. I then immediately salvaged all bunkers, (including the ones on the higher platform,) and barricaded the entrance with supply depots.
If you build as flush to the entrance as possible, you'll notice that 4 supply depots are a very good fit, and that they leave a small opening for zerglings to be able to fit through. Because of AI mechanics, this is where zerglings will attempt to run through, instead of attacking the depots. Score! Build bunkers behind, and more units. Wait it out until the end of the mission.
You can build like this for the get go, but since I tried to get all the achievements (and failed), I tried to preserve the current set of buildings. If you do decide to barricade yourself, it makes for a VERY easy game