This acheivement is for completing level 3 and as such cannot be missed.
The key thing here I found was that if you are going for trying to complete the game without killing anyone you have to be very careful in the first main room. After the woman shoots at you and runs off, run into the room (listen to the recording on the desk and pick up the ID tags under), then go to the doors ahead to pick up the hacking tool. I then proceeded up the left stairs (as the woman and her mates start to come back) and into the vent (there are some goodies in the room below).
After collecting the goodies creep out, left along the balcony and then straight past the stairs. Keep going and in the far left corner is the exit down the spiral stairs (with a save point down there). Sometimes one of the guys would come up the stairs you pass and see me, so hanging back until he goes back down is advisable. It did take me about 4 times to get the timings right, so be patient.
I initially made the mistake of running back to the save point at the start of the level after picking up the hacking tool - big mistake as its then almost impossible to do the room from the entry point with 3 of them patrolling. That said, if you are happy to kill them, use the relay switch at the start to trigger a speaker and then whack one of the guys as he comes out to check. This will thin them out a bit for you.
The other tip is that when you find the flight recorder and get stuck in the room, to get out you need to push the green button to move the racks (you need to move about 3). This reveals the part you need to make your hacking tool work and thus open the door.
The rest of the level is pretty linear (don't want to spoil it), save the level exit where you have to hack the door where you originally picked up the tool. Needless to say, make sure you are stealthy as you approach it, but for different reasons....