Take your time and be patient. Make sure you're using supressors on your weapons so that you can get stealth kills without alerting other guards. If guards have intersecting patrols, wait until they're spaced out before taking one out, and make sure no guards have a line of sight on the one you are targeting. Silenced weapons still make a sound, so the distance is important.
You can even try this online, doing an Army of Two of sorts if you draw an enemy's attention. Your partner can then aggro them and you can pick off the guards while they shoot at your partner.
I got this one on the first mission, just by taking it slow and watching the patrol patterns of the guards. Keep an eye on your mini map, even if you have to rotate yourself around to get a wider coverage on the mini map.