Players spend most of Mass Effect 2 preparing Commander Shepard, the Normandy and its crew for the suicide mission on the other side of the Omega 4 Relay. If Shepard spent their time and effort to secure squad loyalty by helping everyone get their affairs in order, and they spoke to all squad members about upgrades that might aid them in their mission, chances are heading into that final fight didn't seem like so much of a death sentence. Appearances might be deceiving, however, as there are several tasks in the fight with the Collectors that require a bit of finesse, and only certain squad members can do them.
Strategizing how best to utilize Shepard's resources when the time comes means coming back from the Collector Base with all squadmates and crew members alive, but it won't be easy. The pressure of that final fight means making on-the-spot decisions with the power to end lives, and even though the entire squad knew what they were signing on for when they agreed to come, no one actually wants to die during a suicide mission.
Squadmate Resources and Loyalty
Each squadmate comes to the mission with something more to offer than their services (not including DLC squadmates, Kasumi and Zaeed). Armor upgrades, special weapons, shielding and biotic enhancements are all necessary factors for squadmate readiness. The three most essential items to ensure survival come from Garrus, who offers the Thannix Cannon, Jacob, who brings the Silaris Armor Tech and Tali, who can hook Shepard up with the Cyclone Shield Tech. However, each of these items cost resources, so scanning those planets and collecting elements like Palladium and Platinum is an essential part of ensuring those items are added to the Normandy.
Squadmates also have loyalty missions. Because they are putting their lives on the line, it's important to make sure their loyalty is gained before heading into the Omega 4 Relay so they can focus on the mission and nothing else. Attend to the personal missions each one of them asks for help with and carry it out successfully. Not gaining squadmate loyalty is a surefire way to get them killed on the Collector Base.
Obtaining Legion and the Reaper IFF
Once the Reaper IFF and Legion have been obtained, things start to move very quickly. To complete the overall mission, which means rescuing the abducted crew, there can't be any stopping along the way. If there are four or more missions left when the IFF is installed, the game will allow the completion of four missions before launching the Collector abduction aboard the Normandy.
To remedy this, take care of as many of the remaining missions and side quests as possible before going after the IFF and picking up Legion. To gain Legion's loyalty, it must be activated and spoken to before going near the Galaxy Map to gain its loyalty mission, which can then be undertaken before the crew abduction.
Mission: The Omega 4 Relay
After the crew's abduction, heading into the Omega 4 Relay is the priority. Any other stops along the way will result in the crew's death. The items purchased from Jacob, Garrus and Tali will keep everyone alive while approaching the Collector Base. Without the Silaris Armor Tech, Jack could die before even reaching the base. Tali's Cyclone Shield Tech keeps the crew safe while Joker navigates the debris field. The Thannix Cannon protects everyone against the Collector ship. The entire squad enters the Collector Base together. When choosing the squadmates to follow Shepard through to the end of the game, it's important to bear in mind the strength of the defensive team that stays behind.
The first major decision is the tech specialist who navigates the vents to bypass the base defenses and open the main gate. Ideal squadmates for this include Tali, Kasumi and Legion. Anyone else chosen will die before they can open the gate or suffocate inside the vents. To keep the squadmate from dying in the vents, it's essential to hurry through battle and get the next area unlocked. The second major choice to be made is who will lead the second fire team. Garrus, Jacob and Miranda are the best choices, as anyone else will not be able to keep their teammates alive.
Upon discovering the remnants of the crew, who have all survived if no stops were made, reviving them and sending them back to the ship is essential. Choosing a squadmate to escort them is essential to their survival. The most logical choice to take them back to the Normandy (which also guarantees his survival) is a loyal Mordin. Next, Shepard and their squad navigate through Collector swarms, and having a powerful biotic to create a barrier is important. Either Samara/Morinth or Jack are the only options. So long as they are loyal, they will see Shepard and the squad through the swarms without issue. If they are not loyal or a less powerful biotic, like Jacob, is chosen, the swarms will carry off at least one squad member, resulting in their death.
Choosing the second fire team leader establishes leadership for the remainder of the squad, who stay behind to hold the line and provide a distraction while Shepard infiltrates the base. Much like the first fire team, a squadmate with strong leadership skills like Miranda, Garrus or Jacob is ideal. No one else will not be able to handle the pressure or responsibility. Each squadmate has a specific amount of points that determine their defense level based on their loyalty. Jack, Mordin, Kasumi and Tali each offer 1 defense point if they are loyal. Thane, Jacob, Samara/Morinth, Legion and Miranda bring 2 points if they are loyal and 1 point if they are not. Garrus, Grunt and Zaeed add 4 points to defense if they are loyal and 3 points if they are not. Average the entire fire team's defense score and divide by the number of members to determine their overall defense. For example, assuming all squad members are loyal and Garrus leads the fire team, while Thane and Grunt join Shepard, the defense score average is 2, which is the requirement to ensure everyone's survival.
Mission: Reaching the Central Chamber
With a strong fire team in place, Shepard can advance deeper into the base. Five platforms, similar to those on the Collector Ship, fly in with multiple enemies that include Collector Drones, Guardians and Assassins, any of which can become overpowered by Harbinger at any time. Battle through them, advancing when possible to pick up more ammo, and when the sixth platform moves in, take out the Abominations and Scions and advance to the console to fly to the central chamber where the final enemy awaits.
The Collectors have been building a human Reaper, and the big bad at the end is a Human-Reaper Larva that must be destroyed. Between fighting off Collectors, aim at the feeding tubes to shut them down and cut off power to the Larva. With all four tubes destroy, the Reaper falls into the chasm, and Shepard reaches out to the fire team for a status update before telling Joker to prep the ship for departure.
When moving to destroy the Collector Base, the Illusive Man will try to reason with Shepard, telling them Cerberus could use that Reaper to bolster their defenses in the upcoming battle. Shepard's choice here has consequences in the third game. Unfortunately, regardless of Shepard's choice, the Reaper Larva didn't die in the fall. It reemerges from the chasm, and Shepard can target the weak spots in its eyes or chest to begin taking it down. Ammo and biotic powers that help reduce armor are essential here, as they help diminish the Reaper's armor more quickly so it can be taken out.
There are also occasional Collector enemies that join in the fight, with Harbinger taking control of them to try and stop Shepard from ruining its plans. Dispatch them quickly and return focus to the Reaper, which will occasionally drop below the platform and return in a different location. Once the Reaper has been destroyed, it triggers the final cutscene sequences, which show Shepard running for the Normandy to escape the base. After navigating away from the destruction, Shepard can speak with the Illusive Man, whose responses will be different based on whether or not they chose to destroy the Collector Base.
After a conversation with the Illusive Man, the cutscene will visit the surviving crewmembers, showing them all going about various tasks aboard the Normandy. It's business as usual, and for the moment, they are all happy they survived, but the Reapers are still coming, and they'll need to be ready.