The Ancient Rock Wraith in Dragon Age 2 is a really impressive boss, and will probably beat you mercilessly the first time you fight him. Especially if you're playing on Nightmare. This video demonstrates a successful pwning of the Ancient Rock Wraith on Nightmare so you can learn from the creator's success. You'll also see how to use a health glitch in the game that allows you to make this fight a lot easier by taking advantage of teleporting enemies.
From the Creator:
*note on Health glitch*
I found that monsters that burrow or teleport around cause the health glitch. So if you really want to exploit this glitch, wait for the monster to vanish or burrow. Cast regroup, then they should be bugged with health regeneration.