Cuz I do.
I was playing that last level on serious difficulty for the achievement, but then I completed the whole thing in one try without dying or loading a save... ridiculous boss fight included.
Got the achievement for doing that at the same time I got the achievement for beating the game on serious difficulty.
It wasn't my first run at that level. It was the second time I beat the whole thing but the first time I attempted it on serious difficulty.
So who else here was SERIOUS enough to earn this achievement? I want to see who you people are!
The global achievement stats say .6% of people that own the game have this achievement, but I'm going to assume some people cheated and unlocked all the achievements with the Steam Achievement Manager therefore screwing up the stats.