On the mission FRONT LINE you'll need to want to sneak past the first few enemies, go down the stairs, then cross under the connecting bridge. If you turn to the left, you'll see two enemies standing over someone. If you go this way, you can jump over the railing, and find a series of pipes that lead to the ground floor. You can take this route across the map, but watch out for the 4 wire traps. After reaching the end of the ground floor, enter the sewage pipe, and go up the ladder on the right side. Once you climb the ladder, open the door and climb the next ladder which will bring you to a room where there is a guard talking to 3 prisoners. Go up to the guard and knock him out to "save" the prisoners.
Once you take out the guard, the middle prisoner will stand up and thank you, unlocking the achievement. If he doesn't stand up, you may have to kill some additional soldiers upstairs and return to check again.