Quite impressive. I ask because I've only just gotten into achievement hunting here on steam.

Fun question! I had to think for some time, look at the list and such. Dark Souls 2 wasn't a walk in the park but not really that hard . Legend of Grimrock 2 became quite hard the way I did it. I combined a bunch of really hard achievements with permadeath, extremely limited saves and hard difficulty. That was very challenging and super fun!
But generally I don't play many hard games.

what has been your hardest game to 100%?