Since the Prima guide was utterly useless for figuring out the endgame and there seems to be a great deal of confusion on the boards about how this works, I thought I'd spend some time running different teams through the endgame to see what variables are at work.
If the Normandy is not properly upgraded, each missing upgrade will result in a team member death before you even start the assault on the Collector base. If you take this team member with you during the oculus fight, or haven't recruited them, or they are dead, a different character dies instead.
-No Silaris Armor: Oculus beams punch through; Jack dies.
-No Cyclonic Barrier Shielding: Engine core overloads; Tali (or Legion or Thane) dies.
-No Thanix Cannon: Normandy charges in with main guns; Thane (or Garrus) dies.
For the assault itself, there appear to be three types of characters that are required for the various key roles:
-If the Tech Specialist is not one of those listed below, they will die at the first rendevous.
-If the first Fireteam Leader is not a leadership character, the Tech Specialist will die at the first rendevous.
-If the second Fireteam Leader is not a leadership character, they will die at the second rendevous.
-If the Biotic Specialist is not powerful enough, one of your current squad members will be swarmed by seekers and die.
-If the crew survivors are not escorted, they will die (any character can do this and the crew will live).
-If the Heat Exchanger valves are not opened in time, the Tech Specialist will die and you're forced to restart.
-If there aren't enough Combat Specialists left behind to hold the line while you fight the last boss, characters die
-Some characters are worth more as Combat Specialists than others; these point values are still being tested
-Characters with low point values are the first to die if there aren't sufficient points to hold the line
The following characters are the only ones that will succeed at their respective roles:
* Samara / Morinth
* Any loyal character
* possibly other partial contributors
Disloyalty (Normal Loyalty).
Not having a character's loyalty has different effects depending on what you do with them:
-Having disloyal characters in the above key roles will result in deaths, regardless of whether they are suitable or not.
-A disloyal crew escort will sacrifice their lives to protect the crew (the crew will still make it).
-Disloyal characters don't get as many 'points' as loyal characters when holding the line and are more likely to die
-Keeping disloyal characters in your active squad prevents them from dying, except for...
-In the final boss fight, disloyal characters in your active squad will be crushed by rubble after the fight and die.
-If all team members are dead, Shepard misses the final jump and lets himself fall to his death rather than risk Joker's life.
-Note that with all 11 team members recruited, it's actually not possible to kill off all of them and get yourself killed.
Kidnapped Normandy Crew. ( Not essential for the achievement)
If you do not go through the Omega-4 relay quickly enough after the Collectors kidnap the Normandy crew, the captured crew members will die off in the endgame. Watching Kelly get liquefied was particularly horrifying. Who dies depends on how many missions you did after the Joker level (actual clock time does not affect the outcome, nor do relay jumps, fuel spent, or landing on Illium/Omega/Citadel/Tuchanka; any quest, big or small will count though):
-If you use the relay immediately, everybody survives; Horizon colonist gets liquified instead.
-If you do one mission, Kelly, engineer Daniels, mess sergeant Gardner, and half the crew dies.
-If you do two missions or more, everyone except Dr. Chakwas dies.
Legions loyalty mission.
Note that if you activate Legion and learn about his loyalty quest immediately after the Reaper IFF mission, the Collector attack / Joker level will wait for one extra mission before triggering. This allows you to complete all the loyalty quests before the crew is even kidnapped (assuming you did the other ones already).
Some people also report having trouble keeping a character (typically Mordin) alive at the end, where they must hold the line until Shepard kills the last boss. This seems to be related to a choice or choices made during the final mission, as rearranging team roles from the same savegame will affect whether this mysterious death happens or not.
All credit goes to Ken Ueno of the BioWare social site.
(who gave permission for this to be posted here)
His thread.(Link below)http://social.bioware.com/forum/1/topic/105/index/871030