Priority: Earth

Acquisition
This mission is acquired immediately after Priority: Cerberus Headquarters.

The dialogue options to start or postpone the mission do not affect Renegade or Paragon scores.

As noted previously, this mission is part of the end-game sequence. By beginning Priority: Cerberus Headquarters, the player forfeits the freedom of the galaxy and can (from that point forward) not fly anywhere except Earth itself.

Preparation
No side missions available at this point, every system except Sol is unavailable on the galaxy map and no more war assets can be obtained. The minimum necessary amount had been reached latest after Priority: Tuchanka. The effective military strength modifier can still be increased by completing Mass Effect 3: Galaxy at War activities. Having more EMS will result in Earth not taking as much damage over the course of the counter-attack.

Squadmates
Your main priority should be damage output. There are a lot of foes, and many of them have tons of shields and hitpoints. Liara and Javik in particular can be a real liability, because they don't do a lot of damage, can't take much of it either, and their powers are rendered all but useless by the mad amounts of barriers flying around. EDI and Kaidan, who are more intended for anti-barrier work, will do better, but not by much. It's Tali, Garrus, James and Ashley who are your best bet. But you're given a chance to re-select squadmates before the final push, so the first half of the mission can be a good opportunity to experiment.


 * Soldier class: As for this class you probably don't have to focus on bringing raw dps, rather some squad mates with powers. (I have tasted this on Insanity and Liara in combination with Javik works remarkably well despite the describtion above.)
 * Liara's Stasis works agasint Marauders and Warp can take care of Brutes and Banshees, Singularity is great distraction and path blocking ability.
 * Javik provides much needed distraction with the Pull, Lift Grenades and occasional Slam (Dark Channel may be used against Brutes and Banshees thus slowing them down a bit) along with some dmg output.
 * Your job is simple, bring the dps - Incendiary Ammo in combination with Black Widow (Extended Barrel, Enhanced Scope) and M-96 Mattock (Extended Barrel, Magazine Upgrade) should do the trick (You shouldn't need any other gun, although I had M-12 Locust (Ultralight Materials, High Caliber Barrel) thus negative 31% bonus for powers recharge).
 * Just remember killing the stuff in right order. Brutes and Banshees first, Marauders next, and then the rest. Also don't forget use powers as much as you can.

Take Out AA Battery
Hackett and Anderson will walk Shepard through the plan they've put together (which basically amounts to, "Ships shoot Reapers while ground forces try to reach the Conduit"), but like all battle plans, it's tossed out the airlock the moment the Normandy pops through the Relay. Instead of flying straight in, you'll need to stop and take out a Reaper AA Hades Cannon so that the ground forces can get through. Prepare for the fight of your life; this first one is an endless stream of Cannibals, Marauders and two Brutes. Cortez will also be shot down here, which might lead to his death if you haven't taken the time to talk him through his personal problems.

After you finally clear out your LZ, you'll get to advance to what seems to be the remains of a house (if the stove is any indication), where you fight off more hostiles and then get to use a M-920 Cain to knock down the AA gun.

There should be a second Cain nearby. Grab it, and then retreat either back to the west side of the house or to the south side of its eastern half: you need room to use it on the oncoming horde, which includes a Banshee (which may be named Morinth). Just be aware that you can miss, at which point you'll be nose-to-nose with a pissed-off (and presumably radioactive) Ardat-Yakshi husk.

When extraction arrives, don't try to fight: just run for it.

Meet with Anderson
You're now in the Forward Operating Base, where Hammer will assemble before making a move on the Conduit. spread throughout the base you will encounter and have the opportunity to meet however many of the various squad-mates you may have (the number of squad mates can range from as few as three (if you killed Tali and Garrus during the suicide mission and the Virmire Survivor during Priority: The Citadel II) to as many as seven (if you got the From Ashes DLC). You will also run into the current leader of the krogan (Urdnot Wrex / Urdnot Wreav), Primarch Victus, and, via a vidcomm terminal in one of the buildings, Cortez and Mass Effect 2 squadmates (if they are alive).  Enjoy these last moments, as they are basically your farewells to your favorite characters.

Note also that there are a number of one-way doors leading up to Anderson, so don't panic if you haven't found certain characters. Liara, Urdnot Whoever, Javik, EDI and Tali are all on the other side of the rampart and its turret-gun minigame. After you speak to Anderson and make a speech to your squad, you'll have a last chance to pick your squad and make adjustments. Then it's time to face your destiny.

Approach the Conduit
Advance until you see the M35 Mako blow up; there you'll need to swing round its burning hulk and go to the right to reach the street. You'll see another Mako there, which will eventually be set upon by a Harvester. Ignore them; they'll eventually sort themselves out, and in the meanwhile there's a Banshee, a Ravager and other infantry gunning for you.

Afterwards, you'll receive a call from a fellow Hammer platoon that is pinned down by Reaper forces. The door leading to them is in the far left corner, and it leads to a parking garage where husks and two Brutes will set upon you. Focus on evasion and let your squadmates do the damage. Then find the ladder--most easily done by hitting the "Where's My Next Objective?" button, since neither of us have any idea where you'll end up after you're done bullfighting.

The ladder leads to an apartment where a Ravager, supported by other infantry, fires down into the street. Wipe them out to see the platoon advance, and then loot the apartment, which has two medkits and a lot of ammo. Then try the door. It's got a surprise, but a Singularity will handle it.

The street below is a great place to use a sniper rifle, if you have one. Just beware of the Ravager far down the street, and the Brute that will smash its way out of the shop on the far right corner of the intersection. Incidentally, that's your next target. Breaking in through the windows will be easy, but the next fight takes place in its back room, which is a maze of shelves and crates. It's very easy for the enemy to flank from the left. Of course, it's very easy for you to pre-empt that by flanking them first, but that'll expose you to a lot of fire. Having a biotic to send is a great idea.

Once you've made it through this business establishment, jump back out onto the street. You'll see a huge blown-out pit in the intersection with a concrete pillar slanting across it. Run for this pillar: it's the only cover around when the shooting starts. Then continue across the intersection to the business establishment on the opposite corner (evidently a restaurant of some sort). More enemies lurk there, but because you have two windows to attack through, it'll be easy to catch them in a crossfire.

Through the door is the restaurant's back dining room, which is a different question. It's got a big window looking out onto a back alley, where a lot of badguys are going to come from; an obvious solution is to dash to one of the tables and take cover there, but the restaurant's back door is parallel to the door you came through, and enemies can flank you from it if you use the tables for cover. Fortunately, there is a medigel dispenser near the window (here? In a restuaraunt??  Let's never eat here again), which might come in handy when the Brute arrives.

Finally you get to head out into the back alley. You'll be drawn down a long corridor which is remarkably empty. ...At least, until two more Brutes come smashing out through the wall. Retreat and take them out, and then go through the hole they opened, as it leads to a door and a cutscene. Try to relax as you watch it. You will need all the calm you can get.

Thanix Missiles
You'll find yourself on an intersection with concrete barriers strewn every which way, shops on the near corners, a Thanix Missile truck in front of you and another one about thirty meters up from you. Get used to this zone and its three entrances (the left side, the front which faces the reaper, and the right side) because the fighting for the rest of the game takes place here.

The first thing that happens is that your objective changes to "Secure Missile Battery"--so clear the first Thanix truck you come to and wait for the console on the side to become operable. You need to use them against the Reaper, which requires you to link them up with EDI up on the Normandy. The second thing that happens is that your objective changes to "Defend Missile Battery," and reinforcements arrive: first a wave of assorted ground troops out of the front; then another from behind; then a radio message from another element of Hammer company, who tells you that you're about to get hit from the left. She's not lying: Marauders and two Banshees will arrive. There is a M-560 Hydra in the shop on the left, but you might want to save it for later. Be careful: no matter where you're hiding, you're vulnerable from the other three directions.

Once the zone is clear, EDI will announce that the missiles are ready to fire. Considering that there's a second truck of them just up the road, guess whether these ones hit. They don't. The Conduit is distorting their aim beyond even EDI's ability to correct. You're going to have to bait the Reaper into coming towards you and out of the Conduits distortion field.

the most important thing to keep in mind during this next portion of the level is that you have now gotten the Reapers attention. As such, YOU MUST keep a close eye on the Reaper who will be firing it's insta-kill laser at you. The laser will strafe down the street in a reliably straight line, just make sure you are on the other side of the street or bunkered down in a building as it does not seem to cut through the shops. A useful tactic is to pull up your Power Wheel every few seconds, whether or not you need to use a power, and turn your view to find the Reaper's laser. Know where it is, know where it's heading, and avoid it at all costs. It can cause you more Game Overs than every other thing in the rest of the level, combined.

You will now be faced with a squad of bosses consisting of up to six Brutes (who seem to like coming in packs of two and can come from any of the three directions), a Harvester who lands right in the middle of the intersection, and up to two Banshees. The exact numbers seem to vary and may relate to your Effective Military Strength. Kite like the boss Shepherd is or bunker down and burn um. The area is fairly open. Consider using that Hydra on the Brutes, but don't waste it on the Harvester, which can be dealt with by hiding in the other shop (the one that didn't have the Hydra) and using the corner to block incoming fire; in this way, the Harvester is actually the easiest thing to kill. As this is the last time you will ever need your Medi-gel, feel free to use it liberally.

Once you wipe out the boss rush, two infantry squads led by Banshees will begin spawning, in addition to the continuing Reaper laser. The music will also start up, as though suggesting that the fight may have gotten a bit intense. This flow of enemies seems to be infinite, so put more priority on surviving and less on fighting. Try to stay on the south side of the map (near the fired Thanix truck) as much as possible; the enemy will have that much more trouble shooting you. Also, if you go back to the shop the Hydra was in, the laser actually can't hit you, and it's a much more viable spot to fight from with only two bosses flapping about. And finally, consider just kiting: once this phase starts, all you actually have to do is stay alive until EDI gives you the all-clear and you can fire the last missiles.

Once the option to activate the missiles appears, there's no reason to fight whatsoever; ignore any enemies and just run for the panel. Soldiers, Infiltrators and Vanguards may want to use Adrenaline Rush, Tactical Cloak or Biotic Charge respectively. Once you hit that panel, you can chock up one more Reaper kill on your tally.

The Citadel
It's now time to go for the Conduit, but Hammer is badly depleted and overwhelming enemy reinforcements are on the way.

RUN.

Once the cutscene is over, continue to advance. You'll have to defend yourself, but you'll be in permanent slow motion, and lining up your shots shouldn't be hard. Once aboard the Citadel, continue making your way forward; it'll be rather obvious where to go. It'll lead to the final conversation of the game, between you and the Illusive Man.

There are several conversation branches with associated Reputation checks, but the conversation's outcome hinges solely on the final choice. It is possible to fail the final check even if you have a full Reputation bar, suggesting that factors beyond mere Reputation contribute to the check.
 * If you are able to Charm the Illusive Man, he will realize that he is indoctrinated and use his sidearm to commit suicide.
 * The Threaten option will offer you one Renegade interrupt; non-reputation dialogue will give you two. Not taking the final interrupt, whenever it happens, will result in a Critical Mission Failure.  After you take it, the Illusive Man will have a few final words as Shepard opens up the Citadel and allow him to see that which he has always loved best: Earth.

The Crucible attaches itself to the Citadel. Shepard is then given the series' final choice. There are a maximum of three options; however, if the Effective Military Strength of your War Assets is below 1,750, you will simply be assigned an ending based on your decision about the Collector Base. According to outside sources (IGN and GameFAQs), you need an EMS of 2,800 or higher to unlock the third choice.

Enemies

 * Banshee
 * Morinth
 * Brute
 * Cannibal
 * Destroyer
 * Harbinger
 * Harvester
 * Husk
 * Marauder
 * Ravager
 * Swarmer

Aftermath
An ending sequence will run, slightly different depending on your choice. Some scenes will be different depending on your Effective Military Strength; the higher it is, the better shape Earth will be in.

The best possible ending requires an EMS of 4000, provided Shepard has managed to Charm/Intimidate the Illusive Man in the final confrontation or 5000 if you failed, which involves Mass Effect 3: Galaxy at War activities. Other decisions (which ones are not known at this time) will cause an additional scene to depict the fate of the Normandy; one known decision is that following Anderson's example will allow Shepard to possibly survive. Finally, certain decisions (not known at this time) result in a stinger scene in which an old Stargazer tells tales of "The Shepard" to a young boy.

If you select the decision to combine synthetics and organics you will get the Synthesis ending. This option is the third option other than controlling the Reapers or destroying the Reapers. The Catalyst states that it is the best option for everyone though Shepard must submit their own DNA to do it. In this case, Shepard dies.