Most difficult challenge you have set yourself in a video game?

[NerfedFalcon]

Playing Subterranean Animism in the first place. Still haven't gotten up to the actual fight with Yuugi. With anyone.
(That's on Normal, by the way: I don't play on Easy except with the decimal games.)

 

[Argonian alchemist]

GAME: Mindjack

CHALLENGE: Playing through it...

That's not even meant to be a joke... seriously, that was really difficult for me.

 

[Tallim]

I don't do this too often now as I don't have the time. I did once play through System Shock 2 on Impossible using only Psychic powers, this included not using the wrench at all (Wrench only playthroughs are easy). Heart Of The Many was evil simply because range psychic shots have a slow travel time.

 

[bobtheorc]

Starship Troopers: Terran Ascendancy only using equipment seen in the first movie, also restricted the number of officers in my squad to a sensible number. Did not finish, can't actually remember how far I got.

 

[Grotch Willis]

Beating Super Meat Boy, I am stuck on the last level before the final boss and can't get back down from getting the key without dying.
Seriously, i had no other problems with the game until that part.