Collector Base: The Long Walk

The team has split again. One group is moving through an area infested with seeker swarms, using a team member's biotic abilities to hold the swarms at bay in order to reach a security door at the far end of the area. Another group is providing a diversion.

Acquisition
This mission is triggered as part of Stop the Collectors: Defeat the Collectors once you have completed Collector Base: Infiltration.

First Room
This mission starts just after you finish the previous mission. The room you enter has the colonists and the Normandy crew. Shepard walks around and after wiping off condensation from a pod discovers one of two people: The victim in the pod stirs and screams as she is liquefied. Though it is too late for her, Shepard directs the squad to open the remaining pods; depending on the haste the commander chooses to take in arriving at the Collector Base, either the whole crew is saved, half of the crew is saved or only Doctor Chakwas is saved.
 * Lilith will be in the pod if you went to the Collector Base immediately after the abduction of the Normandy crew.
 * Kelly Chambers will be in the pod if you did one or more missions after the crew was abducted.

Chakwas is either thankful or angry with Shepard, depending on how much of the crew is saved, and will explain that the Collectors have been processing humans into a gray paste of genetic material. For reasons unknown to Chakwas, the Collectors send this human paste into tubes and route it to another part of the station. Shepard concludes that the tubes must be followed to reach the objective, but EDI informs you of an obstacle: the only route has too many seeker swarms for Mordin's countermeasures to work. A biotic field is proposed to allow a small squad to get past. Miranda volunteers but the decision is yours.
 * Ideal: Samara/Morinth or Jack
 * Non-ideal: Miranda, Thane, or Jacob.

Once the decision is made, EDI also points out that there is another path that can be used as a distraction. You must now choose a member to lead the second team. Miranda will survive if selected as the fire team leader, even if she is not loyal.
 * Ideal: Garrus, Miranda, or Jacob.
 * Non-ideal: Anyone else.

Once both decisions are made, Chakwas will ask about the crew and their route back. Shepard radios Joker to see if they can pick them up. He says that the crew will have to go backwards, which is not an option. You can choose an escort or not:
 * Ideal: Any loyal teammate that was not assigned as a specialist or fire team leader (Mordin, Tali, Kasumi, or Jack).
 * Sending Mordin may be ideal, because later on, you will leave all team members who do not come with you to fend off an attack; if they are too weak, they start dying (and Mordin, Tali, Kasumi, and Jack are typical candidates, as they have low defense scores).
 * This squad member will not be available for the remainder of the mission.
 * Non-ideal: A team member that you have not secured loyalty from as the crew will survive but the squad member will die.

Once you make the decision, the screen will fade to black and you will regain control. Now you have to select your squad for the mission ahead. You will be facing the same opponents plus husks, scions, and abominations.

Into the Swarm
Save often in this mission because of its difficulty.

The initial part of the mission will be the specialist telling you to go, and moving up until you reach a corner. SAVE here and select the specialist to move on. As you move into the room you will see the swarms outside the bubble and hitting it on occasion. Do not go out of the bubble or you will take damage until you die. Same for your squadmates, however unless you order them they will not leave the bubble.

Be careful, as the bubble can obscure visibility, and the seekers beating against it don't help matters any. Use your crosshairs and bring up the Power Wheel to "freeze" time and locate enemies if you are having trouble seeing.

Stop 1
Eventually you will reach a point where you come under fire, the specialist will take cover, because they cannot fight. Take cover and take out the attacking Collectors. Harbinger will make an appearance and will be the only one to try to run up the ramp to flank you. Make sure he doesn’t because he will be able to wipe out you and your squad quickly. The Collectors will not keep coming, so when you take them out you're done. Save the game and select your specialist to move on.

Stop 2
This walk is a short one because you will come under attack again. As the specialist moves toward the next stop, a few husks and abominations will crawl up to charge the team. However, they will appear at some distance so it will be very easy to take them out at range. Several Collectors will then fly overhead, with one being possessed by Harbinger mid-flight - this is a good opportunity to weaken and possibly take out a couple of the Collectors before they land. The specialist will take cover on a docked platform behind a pillar while the Collectors land in preparation for their attack. As the platform occupies low ground and is protected by the biotic field, it can be used for cover. Take out the attackers and save because the next stop is the hardest. Prompt the specialist and move on.

Hardcore/Insanity Tip: Visibility can be particularly bad at this point, but an alternative is to simply eliminate Harbinger as he systematically possesses each Collector; his flaming body makes him easy to spot, and the fact that he doesn't take cover (and the lower ground you occupy, which negates his cover-breaking attack) will make it a turkey shoot.

Stop 3
As you start to move out the specialist starts to struggle to maintain the field however they are still capable of moving. Eventually you will come under attack again so take cover and watch out from the flanks and from the front. This time you are only attacked by Husks, a Scion and Abominations. The Husks and Abominations will swarm you and your squad, so focus on them first because of the danger they pose. Once the husks stop coming focus on the scion if you haven't taken it down already, and make it fall.

When all the husks are put down, save ( this is the last opportunity to make a manual save for the rest of the Collector Base mission), select your specialist and move on.

Hardcore/Insanity Tip: There is a short wall just opposite where your specialist takes cover that provides adequate cover to protect you from the Scion even if you lean out of cover. If you have the Arc Projector, simply pump bolt after bolt into the Husks and Abominations that swarm you until only the Scion is left, and then eliminate it before it gets close. Your heavy weapon ammo is restored when you begin the next part of the Suicide Mission, so don't be afraid to use as many bolts as it takes.

Long Walk
The specialist is now almost entirely exhausted, strained to their limits by maintaining the field. The specialist remarks that the squad needs to reach the end soon.

This next area offers little cover, like the mission name suggests, is nothing but a long walk. Be careful in this area: you will not have to face Collectors, but you will have to face a few husks and abominations. They will come up from the sides, so whip out the shotguns, if you can, and take them out before they can kill you. Once the door is in sight, the specialist comments again that they need to reach it fast. Now it is a race, as the specialist is limp-running to the end. Once you leap over barriers in the middle of the ramp, a cutscene plays.

The squad races down the ramp and the door is opened by a squadmate who runs ahead:
 * If you have a non-loyal or non-ideal squadmate projecting the barrier, the biotic falls from exhaustion and whichever squadmate is providing a rearguard is carried away by the swarm.
 * If the chosen biotic specialist is loyal and ideal, they will push the barrier back, sending the swarms and Collectors flying, and everyone makes it in safely.

The second squad will radio that they are under heavy attack, then someone will hack the door to allow them in. Shepard covers them and as the leader enters they are shot, Shepard rushes to their side:
 * If Loyal and ideal: The leader shakes it off and survives.
 * If non-loyal or non-ideal: The leader grabs their stomach and collapses to the ground, the shot is fatal and the leader dies in Shepard’s arms.

Once the two teams regroup Joker reports on the status of whoever is escorting the crew back, then informs Shepard that the Collectors are massing outside the doors. Miranda will say that the squad will stay back to cover Shepard and a small squad.

Enemies

 * Abomination
 * Collector Drone
 * Collector Guardian
 * Harbinger
 * Husk
 * Scion