Once you gain access to the "Tunnels" area, after the train station, your objective is to find the NSF commander who has a key for a door that leads to the next area, the helipad. You need to reach and talk to him without killing or even knocking out any NSF combatants.
On Normal this is very easy, as you can mix a bit of running with sneaking, since the enemies both don't do a lot damage and lose you pretty easily. Go to the south end, find the switch to the commander's room behind some cardboard boxes and talk to him. He will surrender, as will all of the NSF troops in the area, so once you're done talking to him, take the Nanokey from his desk and casually go to the other side of this area to progress. The troops will continue to wonder around, but will not attack you.
The achievement will unlock once you're past the door leading to the next loading screen. If the next area loads in and the achievement hasn't popped, you've voided it somehow and need to try again.