Updated 03-27-2016 08:47 PM
This is a secret achievement, and probably the hardest achievement in the game. It's a side quest you will get in the Dark Dwarven Dungeons, after first completing the other 2 side quests from the previous areas for beating a tough opponent (Polar Bear and Orc(?) I believe). He scales to your level, so will have more health as you level up. I still wouldn't suggest attempting this until at least lvl 40, to give yourself some breathing room as far as your health goes. I did it as a lvl 44 Assassin, and found a build that I was able to beat it with first try, after struggling with a few others that failed miserably. First, I used an Epic Manticore Poison that dealt 9 damage/turn, and tried to keep him poisoned the entire time. In my offhand I used an Ancient Flintlock, but it was really just for the starting attack bonus so I could poison him right away. I then used the following spells;
Weaken (kept his mana nerfed to prevent Tail Whip attack)
Stealth (extra shielding/double damage)
Disarm (kept his Bite nerfed/double damage)
Burning Strike
Freezing Strike
I used Disarm right away every time it ran out, kept him poisoned, used Weaken any time his red mana crept up (or to keep turns going/build up more mana for myself), and only used my Strikes when I knew I'd get a 4 or 5 in a row (and/or made sure he wouldn't). As with most of these, a little luck involved, but this loadout worked incredibly well for me against him.