'Skill shot' is related to how fast you swipe your finger across the screen. There is an optimum speed that you must hit. When you begin a swipe to mark an enemy a counter starts, represented by a red and green bar at the top of the screen. If you successfully swipe across an enemy when the red bar intersects with the green, you will land a skill shot that kills the enemy faster and earns you more experience.
Different weapons have different green bar length. The best primary weapon choice to make a skill shots is FAMAS. Using it you will get the achievement in no time, just don't forget to upgrade it.
You will probably get this achievement during your 20-25th missions.