This achievement requires saving 100 civilians in patrol. You can access patrol from level 1 via the waypoint system. Patrol is basically a freeroam area where small events and bosses spawn regularly. For this achievement you need to find 100 people with an orange speech bubble with 3 dots in it above their heads and press A to rescue them. There are usually monsters by them, and sometimes trying to rescue them will start an event where monsters spawn and attack. It's important to note that they will leave if you take too long to rescue them, and I don't think those count.
They seem to spawn randomly, though I tend to find quite a few in the northeast corner of the area. However, they can spawn just about anywhere on the map. It's important to note that since patrol has tons of other players around, it can be hard to find people to rescue, so this may be easiest at night. Legendary quests will often task you with objectives in patrol. Patrol is also probably one of the best places to get gear, so you'll probably be here often. As a result I would suggest not going specifically for this achievement, and instead just rescuing people you come across while you do other things.
LunarKnightX stated that Squirrel Girl has an AoE confuse skill you can use to get the enemies off your back so you can rescue people without having to kill everything, so that's a very useful tip. I know Angela has one too, so Squirrel Girl isn't the only one that can do this.