I don't see all the clamor about glitches and whatnot with Headhunter. Took me about six really good tries to do it. The first time I got the guy on the stair case I got it. Pretty much every other room was a cake walk. I never had it not register a headshot.
Daredevil on the other hand was a bitch and a half to do. And currently By My Hands is giving me a headache. But oddly enough the most infuriating achievement thus far has been Time Management. It shouldn't be difficult. But holy shit did I replay Bend Time Massacre something like 100 times to get that one. I got so good at it I'm 3rd in the world on the leaderboards on 360.