I don't think I mentioned but the second mission of the 2 that you do nets you mercenaries, which are powerful versions of certain units, they are the number one buy you should buy in the campaign, they are pretty much useful in every mission, the only ones that are questionable are the firebat mercs and perhaps the battlecruiser merc, because it isn't that useful in any of the final missions where you can use it..
However onto this mission, I'll just briefly explain the mission and what it introduces. Firstly this mission Introduces Marauders (obviously) and Research Bonus objectives.
Marauders are the backbone of any infantry army, they demolish armored units, this is amplified in multiplayer where marauders also have access to stimpack, making them very powerful in Multiplayer, they are also powerful in single player, especially if you know the enemy has alot of armored units or melee units (later), mostly they are a compliment to your marines in the campaign where you would rather go like 4 marines:1 marauder : 1 medic type armies in most missions, however on hard (note: not brutal) there are no air in this mission so you can go mostly marauders and be pretty good.
The second thing in this mission are the bonus objectives granting you research, you want every single bonus objective because a) you want to max protoss/zerg research asap as they unlock abilities/units/structures that are essential for making the campaign easier and once you max these you gain extra money from bonus research to unlock upgrades. If you want to do achievements and stuff I advise making sure the last playthrough you do nets you all the artifacts you want.
Right, onto the mission basically the mission format is that the zerg is racing you to get to the artifact, the zerg will send units over to attack your base occasionally, the protoss will not besides the first stalker attack. Essentially I think it takes the Zerg around 30-35 minutes to get to the artifact on hard, and around 25 minutes on Brutal, takes forever on normal and casual :P.
Basically, on Hard difficulty or easier you don't have to care about the zerg, you can max your army before pushing if you want and the Zerg won't reach the artifact before you do. On Brutal it's a little harder and you have to move out at around 100 or so food or the zerg will beat you to the artifact.
Firstly Mission Objectives
Main: Recover the Artifact
Bonus: Recover the Protoss Relics (4)
Achievements: Take 0 losses from the Artifact Guardians on Normal Difficulty
Finish the Mission in less than 15 minutes.
Ok to start with you are given 1 Command Centre, 5 scvs, 1 Refinery, 1 Barracks /w Tech Lab, 2 Supply Depots, 3 Marines and 1 Medic. Also you will get 4 marauders via drop-in (3 on Brutal), To start with pump scvs and marines and follow the build order below, remember these are rough guides, how much damage you take from zerg waves will affect your supply rounds, I'll make some comments on specific parts of the build after.
25-Supply Depot
27 - 2 Scvs in Gas*
33 - Supply Depot
39 - Supply Depot
47 - Supply Depot
50 - Barracks
54 - Engineering Bay
55 - Supply Depot
57 - Reactor, Stop Scv Production, Barracks, 3 Scvs in Gas
59 - Attack Upgrade, Bunker
63 - Supply Depot (continuous from now on), Tech Lab, Barracks, Turret
69 - 2nd Bunker
83 - Armor Upgrade
Macro out of your 4 barracks from now on, you don't need an addon on your last Barracks.
A few comments on this build order.. Firstly you may think that putting that putting 2 scvs in gas is actually too much, but 2 means you can build up gas to get upgrades earlier and can have continuous production later, you will definitly need to cut some marauders for marines early on for supply depots.
Your Barracks should be producing 1st: Marauders, 2nd: Marines, 3rd: Medics and 4th: Marines. Note: If you lose your first medic to a zerg attack, produce one immedietly from your 1st barracks or you are going to lose way too many troops.
When making your barracks load them with 4 marines and 1 marauder each, this is probably the most optimum barracks lay down until you get the last zerg research (discussed later)
Also note that you can probably stim on every engagement with your marines, stimming and letting your medics heal them back up can be very powerful. The key thing is, don't save stimpack, use it as much as you want to (you can use it in bunkers).
Last thing is that the turret is there to deal with the 2-3 mutalisks that come, it is optional on hard mode, but you need this on Brutal, perhaps a second and you want to position your first barracks like on the top of your ramp to the left and your second one behind this with the turret right next to it.
Right, depending on whether you want to do the hard achievement you have a leisure of when you want to push out, its nice to push out around like 100 supply if you don't care for the hard achievement, if you do care about the achievement you should move out at around 70-80 supply, you get a couple of auto-saves so if you are late just move out earlier and repeat.
Onto the actual walkthrough though: I am going to move out at around 100 supply, leaving both bunkers full with the above composition and have all 4 barracks rallied to my main force, firstly move down your ramp and grab the artifact there.. move to the west towards the zerg and push up the ramp, there are 2 spine crawlers that will attack you as you move up: attack-move micro works wonders here and just attack the crawlers as you move up the ramp. There is a spore crawler (anti-air) and an overlord here aswell, take them down if you want but not necessary.
Continue moving west and south. Do not go north into the zerg front lines, although you could perhaps take it out, I personally haven't tried and don't want to :D. Once you move off the zerg creep (the purple stuff on the ground) continue moving south till you see 2 cannons, 2 pylons and a lone stalker (3 on Brutal), attack-move micro through them and take the small path to your east and pick up the artifact. Then move back to your ramp.
Ok, once you are back at your ramp you can start to push east, take out the units and the pylons first, taking these down powers down the cannons and they no longer attack. The golden rule is that taking down 1 cannon is quicker than 1 pylon, but 1 pylon is quicker than 2 cannons. Continue moving across the bridge where you will run into a new unit: the sentry, they forcefield you on the bridge, take down the pylons to the north and south of the bridge and then kill the sentries (On Brutal there is a High Templar here who storms your units, move them out of the storm and then kill him asap when the storm ends). Once this is done take down the powered-down cannons and the continue moving east around the path.
You should get to some protoss force at the bottom of the ramp consisting of some stalkers and zealots and cannons, take them out (on Hard mode there are 2 void rays here, stim your marines and take them out asap). There is a small path to the west, go down it and take out the 3 cannons and pylon located here, remember snipe the pylon first after the forward cannon and grab the relic.
Up the ramp is a Protoss base, attack move up there and demolish the force, you will see the sentries other moves here: Hallucination and Guardian Shield, you should focus on the Sentry first so your ranged attacks do more damage to the rest of the protoss army, take out the units first and then the cannons and then the probes (workers) and then the nexus then the rest of the buildings (on Brutal there are archons (and hallucinated archons) up here, they do lots of damage to your infantry so focus them down first).
Once the base is down there is the main bridge to the artifact and to the east of that is a small path with 2 pylons and a relic down here, just go and grab it and then go to the bridge, the sentries again forcefield you off. This time they have stalkers that hit past the forcefields which you can barely hit, snipe off the sentries then charge across the bridge and take out the stalkers.
Now depending on whether you want to do the achievement or not stop or just charge ahead to the artifact with your force. If you want to do the achievement, leave all your marines outside of the artifact area and take only your marauders and medics. When you go over the green thing the GUARDIANS will activate, they are basically reskinned colossii meaning they do aoe damage to your units, hence why you want to leave the weaker marines out of your force if you want to do the achievement, just choose the furthest left statue and surround it with your infantry and focus it down, then the south one and then the east one. If you want the achievement remember to pull back marauders/medics taking damage so they don't die.
Otherwise just attack-move to victory and watch the cut-scene. I was sure the dropship would get shot down since it showed it and had kerrigan saying I WONT UNDERESTIMATE YOU NEXT TIME, but alas it didn't (spoiler?).
When you get back to the ship you should have 110,000 credits and gain access to the mercenaries. Like I said earlier: YOU WANT THESE, hire the marauders for 30,000 leaving you with 80,000.
You now have access in the armory to Marauder tech
You Have 2 upgrades for Marauders:
Concussive Shells - The Marauders' attack now slows the Enemy (note it is in an area around the target unit and not just the target unit) (70,000)
Kinetic Foam - Increases the life of Marauders by 25 (90,000)
Concussive shells makes marauders so useful in a marauder/medic/marine ball in the campaign and so you should upgrade this without question, unless you want to skip marauders completely, if you do I would get the Combat Shield upgrade for Marines for the extra hp, Personally I like marauders in my ground army and they are very useful until you get 25 zerg tech.
Discussion: Because marauders are slow (relatively) to marines/medics a marine/medic ball is almost always more effective in the campaign because it is more mobile, however throwing a marauder or 3 in there to slow melee units definitly pays off. Marines and Medics are the backbone of any army really and adding Marauders means you have a low gas-usage compared to other options, you don't need the extra life on marauders but the aoe slow is soooo useful in balls.
Personally my strat will be a Medic/Marine/Tank/Vessel Army on most generic maps, with the mercenaries mixed in of course. However Marauders are very good till you get tanks and science vessels so I recommend getting these upgrades, and you will get the marauder mercs most games so you might aswell grab this upgrade imo, but getting faster combat shield is just as fine, I will get both.
Please note my strat above isn't that viable in multiplayer because you can't get medics and vessels in multiplayer, Marine/Tank is a reasonable strat, but it requires good macro and micro to pull off, the most similar strategy in multiplayer are MMM (Marines, Marauders and Medivac) armies, which is probably the easiest strategy to pull off, but I won't comment too much on multiplayer, just note it is far more viable in multiplayer because marauders gain stimpack.