The most difficult thing I've ever done in a video game is probably my DLTK run through the Frigate level of Goldeneye.
DLTK is basically this mode where you set enemies' stats to the max, meaning:
-They will kill you with one shot
-Every open shot they get at you hits
-They will always use the fastest reaction animation
-They need to be headshotted 10 times before they die
Another difficulty with the Frigate level is that you start off with only 20 bullets so you have to sneak up on people and slap them continuously while making sure nobody else sees you.
These runs take about 30 minutes to an hour and I died about twenty times before I finally pulled it off.