I was able to get this on the 2nd mission, Subtle Arrow, in Zambia, with 99% Ghost Skill Rating.
With the following tips it is possible on multiple levels, but this is your first easy opportunity.
Recommended:- Veteran or Elite difficulty
- High Accuracy (95%+), Use Mark and
Sync Shot to easily attain this
- Headshots (Aim for the head)
- Side Missions, try your best to complete these during the mission
- No civilian casualties. Try to limit them as much as possible.
- Limit your deaths and checkpoint restarts!
- Use the sensor to scan areas before deciding who to kill.
Remember, if you fail don't worry because chances are you will get it in a different mission.
See the following videos for a walkthrough of the level, and more details regarding the achievement:
Part 1:
Part 2: