I've completed only two, Battlefield 1943 and Harm's Way (which only took about half an hour, at the most. Very easy 200 GS).
I'm only two achievements away from 100% on DJ Hero, though, with those two achievements requiring that I play a game in local multiplayer with two DJ controllers, and that I win 10 online games. The former is difficult because I only know one person in real life that plays DJ Hero, and he's going off to college soon. The latter is impossible because DJ Hero 2's multiplayer expanded WAY beyond what the original was capable of, so not a soul is online now. Hell, it took ages to find a game back before the sequel came out.
I even got the achievement for five-starring every song on Expert, dammit, I'm so close!