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:
From there sprint up the stairs, take a right and double time to the exit.
Knocking out enemies does not void the 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.