In order to get this achievement you'll need to rescue 10 civilians in one playthrough, as progress doesn't seem to carry over. This event happens randomly while you're out and about scavenging. You'll have a notification pop up that says wave 1/2 and you'll have a 4th person in the fight, who is the civilian. Simply finish both waves without them dying to save them. Do this 10 times. This event appears randomly, which means you may not have 10 by the time you finish the game. I'm on my second playthrough now and this seems likely to be another failure for this achievement. If you get all the romance and ending achievements and still don't have this I would suggest a playthrough on casual difficulty where you go out and scavenge every single day so you have plenty of chances to get this.