Dead Age

While traversing Areas and/or working on mission, missions, you'll occasionally encounter extra survivors. They'll join your party for one combat session, but they are generally weak and fragile, and don't really help much. Further, battles with survivors always come in two waves, so they need to survive both waves for you to get credit towards this achievement, so these fights tend to be more tough, even with the survivor helping.

Even given the random nature of these encounters (unfortunately, there is no way to force these encounters), you should unlock this achievement naturally through the course of play. Keep in mind these encounters do not show up in the Desert areas, only on the Highway, in the Woods, and in the City, but since you spend the majority of your expedition time in these areas, this shouldn't be a problem.

NOTE: if you feel you're behind pace on rescuing civilians compared to where you are in game (the game ends on Day 42), feel free to use the First Aid skill (the three points Medic skill I recommend every combat character have) to heal these random survivors during combat to ensure they survive the wave encounters. You should get this achievement naturally even if you don't do this, but it doesn't hurt to speed up the process.