I can confirm it will unlock on easy too, i kept trying on easy as on normal i was having no luck.
1/ Aim for the lip of the helmet/forehead
2/ If it does not unlock go back to dashboard and load in again
3/ Headshot number nine hit the glitched scenery ( a box i was peeking round ) and it never registered as a miss, two more headshots then it unlocked!!!!
good luck as this is the most irritating achievement i have had in ages.
Dont give up!!!