I don't remember which dragon you're talking about. You have to fight many dragons in the game, and most of them are pretty easy.
The only one I had trouble with is an "ancient red dragon". Klauth, I believe. He has a word of power stashed in his lair, and he lives with a bunch of fire giants (which hurt me like hell; I had no fire resistance).
In the last act, you have to fight 2 dragons are once. That's one of the tougher fights I did, but it went pretty well.