This is a tough achievement to earn as you need to be extremely careful as to not be detected. You can be spotted by the enemy, but if you do you must ensure that you kill them within 1-2 seconds.
Take your time as it's a long section from where you start to the server room. I would avoid trying to sneak up behind them and assasinating them as you can easily be detected by other adversaries while doing so, instead use your silenced p99 and aim at their heads.
As soon as you enter the elevator which takes you down to the server room, the achievement will unlock.
You can use the video below which should prove useful as to which path to take. Credit to TheWhiteMages for the excellent video.