Load up the level FRONTLINE
. What you have to do is sneak past all the guards and parkour your way to the bottom of the bridge. I highly recommend you to watch the added video which will show the way as it can be very confusing for first time players.
Knock out the Nazi commander talking to the captives which will unlock this achievement.
In the video it is recommended to also do this achievement:
Take the same path in the video for both achievements.
If you just want to go for the rescuing achievement then you just have to watch until 2:30 min in the video. For the other achievement it has been reported by many that dying voids the achievement and it is best to restart the chapter from chapter select if you happen to die. Getting spotted however does not void the achievement so start legging it towards the exit when they spot you and use a medkit or two when low on health, good luck.