I agree with Jacks, I went in at lvl 11, with my rogue, Dog, Alistair and Lel. Alistair and Lel died early on, but Dog was absolutely amazing. When Dog gets "knocked back", he doesnt actually get knocked back but lies down, so he gets back into the fight so quick its great, and my rogue was happily backstabbing away. My rogue died with Flemeth at 20-25% to go, and Dog kept going and killed her all on his own - he has so many stuns, even at that lower lvl
I think Dog is far underrated by alot of people - it was after that fight that made me take him most places - except Orz when I believed I had to take Shale and Ogh into Deep Roads.
As for the dragon you see during the Urns quest, havent attempt her at all, but from the guides Ive read most of the comments about it on this thread are correct, you need fire resist like you wouldnt believe, and need to be a veteran group, so probably just before Landsmeet.