Mark D said:
Definitely do Feros first.
As for that achievement, well, I wish you the best of luck sir. Apparently you need to complete 95% of the entire game with whatever squad members you want to get the achievement for. My friend has done every sidequest in the game with Liara and Garrus except for 6 of them and still hasn't gotten the achievements. It can be pretty maddening I hear.
D'oh!
Then it's impossible.
I never like to use any one squadmate that much. I rotate them around as needed. Also, I'll pick up the Ashley one on a different playthrough (when I'm not a Soldier myself); there's just no reason to bring her 95% of the time. Some of the time, yes, I'd rather have another combat specialist over a biotic, but I always bring Garrus, Tali, or Kaiden to unlock stuff. I was quite happy to have Tali and Ashley as my rescue team when going after Liara. Tali was invaluable, and Ashley is just damn survivable; Wrex can be nearly as good if you skip out on his biotics, which I didn't.
Well, something to note for future playthroughs, then. Perhaps I'll deliberately NOT pick up, say, Wrex. It looks like you can refuse Wrex, refuse Garrus, and maybe even Tali / Liara, but I doubt you can refuse them all.
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Edit: It's 75%, the same number of side-quests for Completionist.
http://masseffect.bioware.com/forums/viewtopic.html?topic=598842&forum=124
In theory, it should be easiest to acquire for Kaiden and Ashley because you do all the subquests and story quests on Eden Prime with them present. Too late for me though, as I pretty much only had Ashley for a meager amount of time since the Citadel.
It should be hardest for Tali (end of Citadel) and Liara (end of 1 of 3 post-Citadel plot quests).
Well, you only get Garrus / Wrex JUST before you get Tali.
So really, one key is just to plow through the Citadel w/o touching the sidequests, until you have the party members you're trying to unlock "____ Ally" for.
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I kind of hate this sort of Achievement because I like playing with just the right mix for each mission.
The most effective groupings I've found (playing as Soldier) have been:
Wrex & Garrus: Combat heavy but cover all the basics in biotics and tech.
Ashley & Garrus: Sniper team; no biotics, but biotics don't work at long range anyway.
Tali & Liara: Well-rounded group of specialists. All kinds of crazy explosions going off constantly and bad guys flying around. ^_^ This is kind of my ideal party, yeah. I fight and my squad mates handle the "magic," only pausing when I really need a specific skill used in a specific way at a specific time.
Ashley & Kaiden: Kaiden handles all the tech and biotic demands solo, combining Barrier and Electronics to be ridiculous in terms of shielding. Ashley has a lot of Defensive abilities, too, so this team lasts and lasts. The only way to get a team that is more survivable would be to substitute Kaiden for Wrex, but then you couldn't unlock things.