Through the course of the story you’ll come across civilians that are tied up. They are usually easy to find and out in the open. Just walk up to them and hit the to release them. This is a cumulative achievement so you will probably unlock it during your second playthrough.
There's a place to grind this out easily at the end of chapter six on the boat. Before the last boss you'll come to an area where you have to fight a large amount of enemies and get the keys to unlock shipping containers where militia members are held. There's three containers total and seven militia. Rescue the first four and then die so you restart the checkpoint and can rescue the first four again. Don't worry it will autosave the ones you rescue. Make sure to not rescue the last three as that will move the level forward and you'll end up with the boss as a checkpoint.