Just beat it, and I was surprised at the simplicity of defeating the arch-demon. :/
Call in the Steel Golems, have them pre-occupy anything that comes your way while you have your party range it out.

Personally, I used my mage, Morrigan, Allistar and Wynne (gotta have a healer

). If all else fails, had my mage use cone of cold to freeze it up, then either Wynne or my mage used Stonefist to make shards of whatever it was

. Gave Allistar a bow with some exploding arrows, and Wynne and Morrigan, well, they were mop up.

Am I the only one forseeing that the next Dragon Age will have something to do with the Arch-demon's soul?
AND, thought I should give you all this as well.
http://www.escapistmagazine.com/news/view/97905-Dragon-Age-2-Will-Look-Super-Hot