You will given the option at the beginning to go lethal or non lethal, select non lethal for obvious reasons. I even had some guards show as hostile, I believe as long as you can down them before they set of an alarm or get a gun shot off you'll be ok, but I wouldn't risk it, just reload. Guards can be alarmed, as long as they do not set an alarm off, this can be very useful when it comes to knocking guards out and using them as bait. The guards will always take the same path, you can walk directly under CCTV cameras, and always look for the least obvious route like a vent system, not only do these bypass a lot of guards, you get bonus XP. This achievement will just take patience, and being observant. If you think you have been detected at any point, just revert back to a previous save.
