If the rumours are true then reaching rank 100 in gears 2 will be pretty hard.Some people estimate you'll need 6million experience points to reach it.
far cry 2 highest rank one, worth like 600 on trueachievements.com
Hardest one for me so far (which I still haven't got, but determined to do it) is the single bullet on The Orange Box. Get so immersed into the game I just think "ahhh sod it, eat lead muffa" and then realise what I've done!
It's not hard it's just time consuming.
With the amount of lag you tend to get in that game and the fact you may:
1)Not get any experience points if the game selects you as host and your connection drops for just a split second.
2)Lose 1500 exp if you get disconnected for any reason AND not get any exp points the next time you play.
I'd say it might be hard to get.Haha!