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!