Ok I finally got this today.
After trying many different guns, times, and levels I came back on here again to read more about it.
I did this on level 6 (First level of Act 3) on Easy.
Go forward immediately and get the sniper rifle.
Stay back as there is no need to move up. Just start sniping. Take your time and make smart shots.
Like folks said above aim for the lid of their helmets.
I got headshots on the first 10 and the achievement popped right away.
Make sure you see their helmets come off.
Now time to gring 2500 kills to finally put this one to bed