How do I earn "No One Left Behind" with as few squadmates as possible?
Solution 1:
Took me ~2 hours, but I worked it out on my own. For anyone else wondering.
Firstly, check out this post on the Bioware forums: http://social.bioware.com/forum/1/topic/105/index/2933016
All of the information used in this post is drawn from there.
Given Parameters:
Minimum of 8 Squadmates to unlock Omega-4 mission.
Squad includes loyal Kasumi.
Squad includes Zaeed.
Don't let anyone die.
Results:
From my calculations, it is possible to assure the survival of all brought squadmates, as well as the Normandy crew, with just over half the squad loyal to you (8 squadmates, 5 are loyal):
Mordin(Loyal)
Tali
Garrus
Miranda
Jacob(Loyal)
Samara/Morinth(Loyal)
Kasumi(Loyal)
Zaeed(Loyal).
The following 6 characters are required for no deaths. Period.
Mordin -- unlocks science lab and ship upgrades
Tali -- unlocks ship shield upgrade
Garrus -- unlocks ship guns upgrade
Samara/Morinth/Jack -- biotic for "The Long Walk" in the collector base
Miranda -- with you by default
Jacob* -- with you by default
*(If you recruit Samara/Morinth, and you will want to if you're striving for the minimum, you don't actually need to research the armor plating for the Normandy. Jack is not part of your crew, and so she cannot die when the hull is breached.)
It works like this -- stick loyal Tech specialist (Tali/Legion/Kasumi) in the vents and a loyal fire squad leader as the distraction team (Miranda/Jacob/Garrus). Then pick a loyal biotic (Jack/Samara/Morinth) and another fire squad leader (same as before). Lastly, pick a loyal (any) squaddie to escort the crew back. If you made these "ideal" choices (remember, they have to be loyal!) no one will die.
The last two "death checks" are "holding the line" and the escape from the final boss. The latter is simple -- if you take loyal squadmates to fight the Human-Reaper, they will survive. If they aren't, they will die. It is the "Holding of the Line" that is the most complex, and which necessitated both math calculations and prompted this question itself.
Every squadmate in the game has a "hold the line" score, either a 3, a 1, or a 0. Being loyal increases their score by one. If the average "hold the line" score is > 2.0, no one will die. Anything less will result in one OR MORE deaths.
In your case, the situation is as follows:
Tech Expert: Kasumi
Squad Leader: Jacob or Miranda (I picked Jacob, it's preference)
Biotic: Samara
Crew Guide: Mordin
Human Reaper Fight Squad: Kasumi and Jacob
This leaves the following five squadmates to "Hold the Line":
Tali - 0
Garrus - 3
Zaeed(Loyal) - 4
Miranda - 1
Samara(Loyal) - 2
2+1+3+4+0 = 10 / 5 = 2.0 = no one dies.
Without your restrictions on Kasumi (If you didn't want the Locust, or wanted to do Tali's loyalty mission), it is possible to reach the absolute minimum: 8 squadmates, only 3 loyal.
Tali
Jacob/Miranda
Samara/Morinth/Jack
Tali is both your techie AND escorts the crew back. Take Jacob/Miranda and your biotic specialist into the last fight leaving the following 5 to hold the line:
Mordin - 0
Garrus - 3
Miranda/Jacob - 1
Zaeed - 3
Grunt (instead of Kasumi. Don't recruit Kasumi) - 3
3+3+3+1+0=10 / 5 = 2.0 = everyone survives.
Solution 2:
Honestly, I think the best approach to get this achievement is to do all of the loyalty missions.
Unfortunately I can't find the link now, but there was a poster (created by GamePro, I think?) that listed all of the logic that the game uses to decide who lives and who dies.
At a minimum, you need the following:
- A second team leader (I know this can be Garrus or Miranda, maybe Zaeed, possibly a few others)
- A tech expert (Tali, Legion, and (I think) Kasumi)
- A biotic expert (Jack, Samara, or Morinth)
I think the person you have escort the surviving crew back to the Normandy must be loyal for all of the crew and the teammate to survive, but I'm not positive on that.
Each person you pick for these tasks must be loyal, or else they will die.
It gets tricky when it comes to the team that you leave behind to defend while you take out the end boss, A teammate can survive this if they aren't loyal, but their odds greatly increase if they are loyal (in one of my play throughs, Legion wasn't loyal (I sided with Tali in their argument), and he still survived; everyone else was loyal though). Leaving strong teammates behind to help defend (Grunt, Zaeed) also helps others to survive, but they must be loyal to help increase the odds of others surviving. For some reason, Mordin tends to die here, even if everyone is loyal, so I'd either take him with you for the final fight, or have him escort the crew back to the ship.
Once you've completed the final fight, I think that the teammates that you took with you can die if not loyal, but I'm not sure if they have to be loyal to live.
Basically, if you want to be sure, just do all of the loyalty missions. They aren't that long, and the story is good enough that they're interesting to go through again (at least for me).
Update: I found the poster I was thinking of, thanks to Beska in the comments. See How To Save (Or Kill) Your Party In Mass Effect 2 at Game Informer for exactly how your party members live or die in the final mission.
Solution 3:
You must do all the ship upgrades.
While the loyalty missions are not all required, it's just easier to do 'em all. If I recall correctly, though, you can have a total of 2 people not loyal and everyone still survive. But, those people must remain with you in your party pretty much at all times.
As long as you do all the ship upgrades and the loyalty missions and don't be stupid with your leadership choices, you'll be fine. I was stupid with my final leadership choice and sent Legion as the head of the other group. He was the only one that died on me.
Solution 4:
It is also the case that people can die randomly during the mission...though the chances of this can be greatly decreased by being loyal and by making correct selections for the leaders of the sub-missions.
It is highly recommended that you send Mordin Solus, if you're going to send anyone, back with the rescued crew, as anyone you send back (who is loyal) will survive, but he stands the best chance of dying randomly during subsequent encounters.
As an aside, you can keep everyone loyal, despite potential problems, which will also help. Some people will fight with each other (Tali/Legion, Jack/Miranda), and when you take one of their sides, you can potentially lose their loyalty. However if you have gotten your Paragon or Renegade skill high enough, you can take that option, and save the loyalty for both of them. I've also heard that you can regain their loyalty later, if you get your skill score even higher, though I found that very difficult...it's much easier to just do it right the first time (which means having a high score, which means doing a lot of side missions first, which kind of avoids the result you were trying to achieve.)