I've been trying for this achs for a few hours now, and I just noticed something, the Nazi'a helmets pop off no matter what, I shot one in the foot, and his helmet still popped off.
So helmets coming off have nothing to do with headshots.
Edit: Oh thank god! I finally got it in the third level, after getting a headshot on the last sniper.
Last edited by Earthbound_X; 03-06-2011 at 02:47 AM.