The one who requires you to have one soldier survive every mission. Do you have to have him or her on every mission with you? So if he or she can't make it to one because of an injury you're boned?

Because I had the dude who survived the tutorial mission almost always with me and he became the volunteer in the end. Still didn't get the achievement though.

@pyrodactyl: Yeah, I mean technically you can ignore missions, so if you know your one guy is wounded you could always just ignore missions until he is healed, and that way he won't ever miss out on a mission. But that's seriously a bad idea because your panic levels will take a huge hit every time you do that.

So yeah, I'm guessing that soldier needs to literally be on every mission you ever do. That achievement seems pretty rough.

@pyrodactyl: Yes, if your key soldier is injured and miss a mission, you'll lose out on the achievement. So not only do you need to keep him alive through all missions; you also have to make sure he's never injured for long periods of time (since if you pass on too many missions the panic levels will rise too much). A playthrough on Easy with saving between every turn is probably the easiest way to get this achievement...

i think it is that it should survive a single playthrough but im playing on xbox if your on pc it may be different