I dunno... I don't think most achievements are very difficult if you're patient. Most of them just require a lot of tedious effort, or a specific sort of playthrough. I feel kinda sorry for people who can't get the "Asari Ally" achievement in Mass Effect. Some people still think it's bugged, but you can actually get it before reaching Virmire if you know what you're doing (if this is giving you trouble, PM me and I'll explain how to do it... I've never actually seen a walkthrough that explains how to get it as early as I did).
Nearly all of the Halo 3 achievements are reasonably easy, but some of the multiplayer ones are really awful (usually the ones that unlock Elite armor, because Bungie hates Elites I guess).
Condemned has an achievement for beating the game without using firearms (melee combat may be a core mechanic of the game, but that doesn't make it any less brutal against some of the tougher foes). I haven't gotten it yet, but it sounds like a good way to get ready for Condemned 2, which I don't own yet.
I think the worst one I can think of was "Hella Copter" in Dead Rising. To get it, you basically have to get shot up by rockets and machine guns while aiming, WITH THE THUMBSTICK AND NO AIM ASSIST, at the tail of a moving helicopter... and repeat this until you do enough damage. That game also featured the "Saint" achievement, which some people just can't put up with the survivor AI (or lack thereof) long enough to unlock.
Not neccesarily hard... terribly easy, in fact... but my stubborn determination to get to the next fucking checkpoint in a CoD3 level got me the "Purple Heart" achievement... probably the only achievement which has ever made me laugh out loud. You basically have to get yourself killed OVER AND OVER AND OVER, I'm guessing a few dozen times, trying to accomplish the same thing. I must have died 20 or 30 times between one checkpoint and the next before I got it (it's a secret achievment, so I was totally surprised).
