I just got this achievement. I got it on Mission 6, 'Storming the Tower' (the only where you land on the rooftop).
I took the advice from comments posted here and on Trueachievements and ignored the turret gunners.
- I didn't have any cheats activated.
- I unlocked this on Easy.
- I don't know if this played a part or not, but when I loaded the level from the chapter select, I did not skip the cutscene (the news broadcast). I just let it play through.
- I aimed for the general helmet area, not the face (some were hit at the very top of the helmet, some along the brim area, but I never aimed directly at the face).
- I didn't miss any shots.
- I didn't manually reload at all; I let the gun reload after running out of bullets.
First, I shot the enemy that was on the walkway on the left side from where you pick up the Gewehr 49 (the one that throws grenades that keeps ducking behind cover). Then I shot all the enemies that appear at the back right (by the door and turret, I think three of them). I also picked off a couple of guys that were on the above walkway on that side. By that time, another guy had appeared on the left walkway, so I shot him.
Then I moved left and got the guys that were at the back left (maybe three of them, and again, ignoring the turret gunner) and a few guys on the above walkway. So on that side, there were maybe six enemies total. The only remaining enemies were the two turret gunners, and I've killed about 12-13 enemies as of now.
I waited around for about 30 seconds (taunting the turret guys) and it finally said "Objective Complete". Then they started pouring out of the door (about 8-10 of them total in waves of two). The achievement unlocked on the last enemy that had come out of the doorway. I didn't shoot the two turret gunners at all.
It was definitely more than 10. More like 20 or so. Either that or I missed a headshot (damn hit detection) and the count restarted early on, but every shot hit something and I swear I hit the head with every shot. From what I could see, about 95% of the helmets came off. There may have been a couple that didn't, but it was hard to see the guys on the above walkway since it's darker up there. Every guy's helmet in the courtyard definitely flew off. Again, helmets come off if you shoot a guy in the foot, so I really don't know if that has any bearing on whether the game counts it as a headshot or not.
I had to reload the checkpoint only once today (I've been trying on and off for over a year to get this achievement).
Playing on Easy gives you unlimited time to line up your shot. Just take it slow and hopefully this achievement won't take long.