When you are playing an Airborne Objective or Airborne deathmatch game and you are on the Allied team, you can try to get this achievement. At the start of the match or after you are killed you will spawn from a plane and you will have to parachute down. If you see an enemy on the ground, try to move towards them, and when you are right above them, do a melee attack (default button for this is pressing the right stick). If you hit them you will instantly kill them and you get the achievement. Be careful because you will be an easy target if you are aiming for the enemy on the ground.