This achievement is a pain in the butt because it's so time-consuming but otherwise easy. Some quick tips:
- Lower difficulty to Story
- Equip "Lightning Reflexes" and "Anatomical Knowledge" abilities from the Combat branch. Also the "Steady Shot" ability from the General branch.
- There's a place in the south east of Skellige where there are 2 infinitely respawning pirates. You can kill them and they'll respawn after a few seconds and kill them again. I found this much easier than Devil's Pit.
- Make sure your screen shakes for a split second you headshot kill them. That's how you know the game registered it as a headshot kill.