Before launching the Suicide Mission, make absolutely certain that you have purchased the armor, weapon, and shielding upgrades for the Normandy, as for each one of these not purchased a squad member will die during the run to the Collector base.
The following list shows, in order of preference, which crew member will die if you haven't upgraded that part:
Heavy Ship Armor: Jack
Multicore Shielding: Kasumi, Legion, Tali, Thane, Garrus, Zaeed, Grunt
Thanix Cannon: Thane, Garrus, Zaeed, Grunt, Jack, Samara/Morinth
Note: This tier is sorted by the characters' order of dying in descending order. Squad members on the "Multicore Shielding" list will not die if they are in your party when you fight the Oculus. Bringing party members to fight the Oculus will not protect Jack if you do not have Heavy Ship Armor, or anyone on the "Thanix Cannon" list.
After entering the Omega 4 Relay but before the beginning of the suicide mission, your ship will be attacked by an Oculus at two different intervals (between cutscenes). You will be taking in a team to fight the Oculus, though the team selection does not have any consequences or impact on gameplay, thus you may choose anyone you want.
There are three stages to this mission, during which you must select members of your team to perform certain duties. Choosing an inappropriate character for those tasks will result in someone's permanent death. Appropriate choice are listed after Ideal, while inappropriate choices are listed after Non-Ideal.
Note: If an ideal squad member is not loyal for your playthrough, consider them to be a non-ideal choice.
If at the end of the final battle only one or no squad members survived, no one will help Shepard onto the ship, resulting in the Commander's death. A single surviving squad member is not enough to save Shepard.
Collector Base: Infiltration
A Specialist Technician is required to infiltrate via the station's vents to bypass the defenses and doors then open the central gate for the rest of the squad.
Ideal: Tali, Legion, or Kasumi.
Non-Ideal: Anyone else, or a disloyal technician, as they will be shot in the face while forcing the door shut.
A Fire-team Leader is required for the second team. The briefing conversation mentions that this member must be experienced in leading a team.
Ideal: Garrus, Jacob, or Miranda
Non-Ideal: Anyone else, as they will get your specialist technician killed, even if a loyal Tali, Legion, or Kasumi was chosen.
Your Team – will be fighting against Collector Drones, Harbinger, and several Collector Assassins, all of which have biotic barriers on harder difficulties (Drones gain barriers on Hardcore and Insanity only). Characters with Warp, Incinerate, and Concussive Shot powers are a good choice. Warp and Concussive Shot are effective at taking down biotic barriers. Warp and Incinerate deal damage against armor - with Incinerate dealing decent damage against organic enemies along with setting them on fire for a few seconds, and it does some damage against barriers.
Miranda, Grunt, Samara, Zaeed, Garrus, Thane, and Mordin are all good choices since all of them posses either Warp, Incinerate, or Concussive Shot which are good powers to have for this segment. Samara can unlock Reave, which is a lot like Warp except it can heal Samara and give her a temporary health bonus, not to mention it can damage a Collector standing behind a guardian shield.
You will need to fight several Collector squads, while pressing forward to activate eight different consoles alongside a path beside the vent in which is your Specialist. The heat in the tunnel will kill your Specialist if you take too long to reach the consoles, so you should clear out the enemies as quickly as possible. Another tactic (although it might require a few tries depending on your skill level and the difficulty setting) is to ignore enemies and simply rush ahead opening all the consoles in your way. Also, while the Collector Drones fly in to engage your squad, they are highly vulnerable in mid-air to Throw, Slam and Concussive Shot, in short any physics damage attack except Shockwave and Pull since the former will not hit airborne enemies and the latter has very little effect on them. In normal difficulty, this will instantly kill the Collector Drone and Throw Field can take out an entire group before they get a chance to attack you. It is advisable to save your game before you press the last button in case you select the wrong character for the next session. The mission ends immediately once the final button is pressed, so if time is near to running out completely it may be worth it to ignore any remaining enemies and simply run to the button and press it.
Collector Base: The Long Walk
A Biotic Specialist to generate a shield, protecting the squad against Collector swarms.
Ideal: Samara/Morinth or Jack.
Non-Ideal: Anyone else. Doing so will result in one of your squadmates being carried off by a Seeker swarm.
A Fire-team Leader for the second team (referred to by Miranda as "the diversion team").
Ideal: Garrus, Jacob, or Miranda. Note that Miranda will not die here even if disloyal.
Non-Ideal: Anyone else. There is a possibility of this leader dying as the doors close at the end of the long walk (shot in the stomach as the door closes).
Note: Even if you choose an optimal candidate, they may still be shot (and play the same initial animation). DON'T PANIC, they will survive the injury. This is confirmed when Shepard comes to their side and they remain standing.
An Escort to guide the Normandy crew back to the ship. However, you can't use this person for the final battle. Choosing no one results in the death of the Normandy crew.
Ideal: Mordin. This will ensure his survival because he has a strong chance of dying if he doesn't escort the crew back, even if he is loyal. Otherwise, any loyal squad member is guaranteed to successfully escort the crew back to the ship.
Non-Ideal: A team member that you have not secured loyalty from. This squad member will perish while escorting the Normandy crew. Also, a squad member with a high "defensive skill" value (Grunt, Zaeed, Garrus), as this may indirectly cause Mordin's death if he is left to hold the line.
Your Team – You will be fighting against Drones, Harbinger, Assassins, waves of Husks and Abominations, and a Scion.
Tali and Legion are probably preferable for this segment; their combat drones can escape the biotic field safely and root out any dug-in enemies. Kasumi is a bad choice as her shadow strike ability leaves her vulnerable to the seeker swarms.
Another strategy might be taking some good damage dealers. Jack, Zaeed, Grunt or Thane will do much more damage, especially important for husks!
Your chosen Biotic Specialist will erect a dome-shaped biotic shield encapsulating the squad from Collector swarms. Note that moving outside of the biotic barrier will cause damage to Shepard and eventually result in death. The Biotic Specialist will move slowly through the level and stop three times for you to fight off a wave of enemies before proceeding. The waves encountered are:
Drones and Harbinger - on the far side overlooking a small pit with a large block in the centre.
Husks and Abominations climb up from a ledge then Drones, Harbinger and Assassins will fly in - on a long walkway with a left-facing 'sniper alley'.
Husks, Abominations, and a Scion - at a left-turning bend in the road with a raised platform on the left side.
Final Battle
Note that, while manual saving is not allowed from this point onwards, the game will auto-save at the beginning of each part. Loading an auto-save also gives you the opportunity to change the weapons loadout of your squad. If you die, you will have full ammo when you reload the game, even if you didn't have full ammo at the last save.
Part 1 – Team Selection
The team you select here will be with you for the remainder of the suicide mission – you will be fighting against Drones, Harbinger, Assassins, Abominations, Scions, and one giant Human-Reaper larva.
Ideal: Squad members who are loyal to you and are less suited for defensive tasks such as Mordin, Jack, Kasumi, and Tali.
Non-Ideal: Squad members who are not loyal to you. They will die after the battle as the Collector station begins to fall apart.
Note: The teammates you do not choose are automatically assigned to hold the line against the Collectors. A 'defensive skill' value is calculated for the 'hold the line' group as a whole, based on a defensive value inherent to each character. Loyal characters gain a bonus to this value. If the 'defensive skill' check is not high enough to ensure all characters' survival, squad members will be designated to die based on their vulnerability, with Mordin taking priority if he is present. Leaving behind strong defenders, such as Grunt, Garrus, and Zaeed will increase the group's odds of survival. In contrast, certain characters such as Jack, Tali, and especially Mordin have less skill in holding their position and should be removed through escort duty or taken along. The entire group will survive as long as the combined defensive skill is high enough, otherwise the game selects a squad member to die based on the scenario of who was left behind and how loyal are the group members. (Squad members who are not loyal are most likely to survive here than anywhere else, provided the group's collective defense skill is high enough, although they will be the first to die if the skill is not.) Leaving behind two strong defenders such as Garrus, Grunt, or Zaeed should suffice, provided you haven't lost too many people.
Note: The order in which squad members die is as follows: any non-loyal squad members, Mordin, Tali, Kasumi, Jack, Miranda, Jacob, Garrus, Samara/Morinth, Legion, Thane, Zaeed, Grunt
Powers must be used efficiently as Ammo is very scarce and must be scavenged whenever possible. Platforms will fly in, carrying several Collector enemies on them. The last platform will contain three Abominations and two Scions. After defeating them there will be a console to trigger the next section. Like the similar battle onboard the Collector Cruiser, using abilities that get enemies off their feet when the platforms are flying in can be invaluable as it prevents them from reaching you, and allows you to pick them off from a distance.
Note: On all versions there is a glitch where the two Scions will not attack until you run up to the platform to engage them. If you have a sniper rifle or Warp you can kill them from 2 platforms away before they are aware you exist.
Note: Whichever Heavy Weapon you bring with you will have its ammo maxed out as soon as the player takes control.
Part 2 – Destroying the Feeding Tubes
More fly-in platforms with various types of Collectors. After clearing each platform, the armor covering the four tubes of the inactive Human-Reaper will open for a short period of time and should be destroyed immediately.
It is possible to destroy at least two each round. The M-15 Vindicator or M-12 Locust can both be used to destroy two at once without powers. With heightened Adrenaline Rush, the M-6 Carnifex hand cannon or the M-5 Phalanx heavy pistol can destroy each tube with one shot. The M-76 Revenant machine gun has sufficient ammunition to destroy all tubes in one long burst, finishing the battle in one round. The M-920 Cain can also take out two tubes simultaneously. The Collector Particle Beam can destroy all four tubes in just one go before the covers come down. It is also possible to destroy the tubes or at least heavily damage them by using Warp, so having squad members with this ability can be useful. Alternatively, if all else fails, you can instruct squad members to attack the exposed tubes while attacking a different one yourself to take down 2 tubes with standard weapons.
Destroying the four tubes will cause the Reaper to fall into the chasm below. Shepard will open up the platform's core and prepare to plant a bomb, but will be contacted by the Illusive Man, who asks Shepard to preserve the facility both to use against the Reapers and to "secure humanity's future against the Reapers and beyond." Shepard will then need to make a choice -- destroying the facility is a Paragon action, while preserving the facility for Cerberus to use against the Reapers is a Renegade action.
Note: During this conversation, Shepard might refer to team members giving their lives even if the entire team has survived up to this point (depending on the selected dialogue option). Also, if Miranda is in the party, a few extra lines of dialogue between her and the Illusive Man can be heard.
Part 3 – The Human-Reaper Larva
You start behind a good barricade spot and you can begin shooting at the Reaper's eyes (they are small, but there are 3 of them, two in the left socket, one in the right), or the large weak point on its chest. It has a nasty charge up attack and it would be wise to not expose yourself until it is finished. If you are hit by it, your shield will be depleted and will take longer than usual to recharge. (If you're feeling brave, you can also take advantage of the fact that it stays still while charging its attack to get in some reliable damage; listening to the charge-up sound will tell you when it's time to get back behind cover.) Be aware that the Reaper's glowing mouth is vulnerable to your weapons while this attack is charging, and presents a very large target for gunfire. Note that you can fall off the platform you are on, so stay away from the edges.
Once you have damaged the Reaper enough, Collector drones will attack. During this section of the fight Harbinger will drop special Heavy Weapon ammo packs when killed. Also note that the Reaper's mouth weapon will injure or kill any Collector enemies it hits. When the boss' life is below 25%, it will begin dodging around and send more attacks at you. Continue shooting at its eyes or use the M-920 Cain, directed at its chest and the fight should be over shortly, leading to the end of game cutscenes. Sniper rifles are surprisingly effective: when the Reaper leans to the right there are about 3-5 seconds worth of reliable targeting. After the first shot, an Adrenaline Rush (if available) allows for a second reliable shot. This has the advantage of spending very little time outside of cover (just enough to fire two shots), allowing for little to no damage to Shepard.
The target is armored, so sniper rifles get a bonus. Widow and Mantis rifles have a 50% bonus against armor, and, boosted by rifle damage upgrade (50%) and heightened adrenaline rush (100%), the damage is massive. With perfectly aimed shots (it's not difficult to aim for the single eye on your left when on adrenaline), the Reaper can be dropped in four or five shots on Casual using the Widow with zero damage to Shepard.
Note: At the end of the battle when you talk to Joker via comm link, he will tell you all "survivors" have made it back to the ship, even if you saved your entire team.