The Witcher 3: Wild Hunt - Complete Edition

This achievement is a pain in the butt because it's so time-consuming but otherwise easy. Some quick tips:

1. Lower difficulty to Story
2. Equip "Lightning Reflexes" and "Anatomical Knowledge" abilities from the Combat branch. Also the "Steady Shot" ability from the General branch.
3. 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.
4. 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.

-For this particular edition of the game, if your achievement is not popping after 50 headshots, hard reset your xbox and start up from your save point. After 6 hours of getting headshots (as well as the other combat achievements), I hard reset my xbox and got each combat specific achievement after one kill. This will save you tons of time.

-Also wanted to clarify this means 50 human OR nonhuman enemies. You only need to kill 50 of one type. I would suggest killing the bandits at Devil's Pit in Velen. You can lure them to the door and they will not be able to come through. This allows you to aim at their head with a fair amount of accuracy.

EDIT: According to Cheva1ier, the bandits pursue you past the Devils Pit. Awaiting confirmation from others. Just wanted to let others know just in case

On Death March, NG+, or Death March and NG+ this can be a pain to unlock. One potential solution is to save this to the end of the game, after reaching a high level and gaining Geralt of Rivia's Crossbow from Touissant (or a high damage alternative from the vanilla game). Returning to Velen or White Orchard you'll find that your crossbow shots do drastically higher damage and should make short work of foes, especially if you have the Lightning Reflexes perk allowing you to slow time while aiming. This process is made even easier if you use the Cat Eyes mutation perk from Blood and Wine, allowing you to essentially one-shot many foes.

If you're like me in clearing the game but finding it difficult to come across groups of low level bandits, you can also do this with high level enemies. It takes substantially more work and is something of an unofficial challenge, but it's possible to complete by using Lightning Reflexes, Cat Eyes, special bolts (like Split Bolt or Tracking Bolt) and a decent crossbow.

Note! I strongly suggest avoiding any bolts, crossbows or potions that induce damaging effects on foes while going for this achievement. Often you can have your headshot stolen by some errant poisoning or bleeding effects before you can land the killing blow, essentially wasting your time and forcing you to try again. An alternative would be to face your foes in regular combat countering and slashing them down to around 20% of their health, allowing any potential status effects like poisoning or bleeding to subside. Then you stick on Quen, aim your crossbow and constantly move around your foes while killing them solely with headshots, preferably with standard or split bolts.