You completed Dungeon Siege III at the highest difficulty level.

This is the hardest achievement in the game. The simplest way to get this is to find a friendly gamer who has a save at the end of the game on this difficulty and beat it with them. Overall, this just requires some dedication to beating it, and accepting that it will take you many hours. The only battle that I consider Near to impossible is the first fight with Jeyne, as you get overwhelmed way too quickly. Practice, practice, practice! Remember to level up if you are finding something too hard.