Cloudkill makes wonders but I believe it is not possible to kill this beast simply standing somewhere out of his visual range and casting Cloudkills - the dragon can heal himself. However this spell is VERY useful against most of dragons - when I had to kill Firkraag early in the game (needed something for my Undead Hunter
, mages used cloudkill and hasted PC distracted dragon running around spell-affected area trying to keep dragon within... Of course, it was not all the job - there were monster summoning too, and archers did their best, and it was not simple, but... eventually he died.
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Tanda,
with best regards from Guild of Assassins