A lot of the time when I see an Achievement description for a certain difficulty, it includes wording such as "without changing the difficulty after starting the game." The 100-point Achievement for The Witcher 2 just says to finish the game while the difficulty is set to Dark mode.
It's probably too early for anybody to have tested this, but I'm wondering whether a complete Dark playthrough is implied, or if the Achievement description can be taken at face value -- just set the game to Dark mode before beating it.
I started on Dark and have left it there into Chapter 1 (in the forest town, beginning a few quests), but with the tutorial telling me I ought to be playing on Normal, I worry that it's going to get too tough to handle sooner or later. Anybody farther along on Dark difficulty enough to attest to how much the challenge ramps up?