they'll be a level early ingame where you have to save someone from a school. You'll see the compound from high up on a cliff and the guy'll say you can shoot the tanks around a fuel depot to get a distraction.
Around the cliff top where you are is a gun with a zoom scope. Pick this up, set your rifle to single shots and test without a silencer (a silencer reduces damage but there are also some patrols that may get alerted to you).
The cheevo is NOT for alerting them, but not being seen so having a ton of people around you on yellow status shouting i hear shots etc wont effect you unlocking it.
You should be able to kill a good 10-15 guys around the compound entrance and unlock the achievement along the way. It doesent matter if they go yellow, run around saying i've been shot etc, aslong as they dont go red and shoot at you, its all yours.