Dying Light

So one really helpful tip I found to getting this achievement with relative ease was to have Rais' men trying to hit me with their melee weapons. It doesn't count if they're unaware or throwing knifes at you, atleast in my experience. I've killed about 20 guys and only had one recorded headshot, then I let them try and hit me and the next 8 of 10 kills counted (I'm not the best shot). Also if you're in the post-game I would suggest reverting your game back to one of the first missions to allow for more of Rais' men to spawn. Hope this helps you out and if you have any other tips on getting the guys to spawn let me know in the comments and I'll update the solution. This is my first guide so any advice or constructive feedback is much appreciated.

Alternative (Povided by WizardSleeve08):
"If you've already completed the game, go to chapter select and choose "Siblings."

You'll need to go meet Jade, skip the cutscene, then find your way into the school. Make your way through the school until you enter a door and encounter Rais's men.

You'll get a checkpoint and three guys to score headshots. Do not go any further or turn off the alarm after they are dead! Pause the game, quit to main menu, then choose to continue your game. It puts you right back outside the doorway with those same three guys."

Thanks for the tip!

Here a easy method I got this at 100% game completion.

-Go to Old Town

-Look for The Stadium safe zone (far left middle)

-Go two buildings over to the right on map (there should be a get inside door)

-In the alley down below is a air drop spawn if you see red triangles you have 3-4 Rais

-if they aren't there go back to the safe house sleep till the next day go back and check.

-repeat

Tips...
*Bring med packs and plenty of arrows
*it takes 3 headshots with a X2 king bozak bow
*make sure you are level with them shooting from the rooftops will penatrate the head and go into the body counting as a body shot
*zombies will attack you and Rais men don't let the zombies kill them
*if you get fairly close then back up the ramped portion of the alley, leading into the square, if only one chases then he normally glitches standing still where the ramp begins.
*use regular arrows

If anyone can shed some (dying) light for me 😅 lol at dying hahaha

I found this out only needing two sets of spawns and did it twice more and got them everytime. My I'm game clock is at 7:00 don't know if that matters, or if it's a set spawn upon loading into the game because I remember in the past not having these events everytime. If anyone can confirm if it's in game time or if it's just if they are there then they spawn there after every sleep period.

If you have already 100% completed the story and are having issues (like I was) finding Rais's men. This will help you find them. Go to Old Town and search for the "Air Drops." Air Drops can be found with an icon on the map that looks like a hexagon with an arrow pointing down in the middle. Not every Air Drop will have Rais's men, but I was able to get this achievement without much effort using this method. Keep in mind Rais's men will be wearing gray and yellow attire. If you headshot the guys wearing black beating up survivors they will not count towards this achievement. If you play on normal difficulty, it only takes one arrow to the head to kill one of Rais's men. After you kill them all, simply search for another air drop and repeat. I'm not saying this is the quickest method, but this really helped me after beating the game because Rais's men are harder to find after completing the story 100%. Hope this helps!