This achievement can be obtained by playing sniper, on any map and gamemode.
The achievement requires to kill scout with your sniper rifle or Huntsman Bow. It's really easy! Just play a lot of sniper, on 2Fort, and when you see the enemy scout on the bridge's roof and shoot him in mid-air. There is a lot of scouts on 2Fort server.
The achievement will be unlocked automatically after killing scout in midair.