Yes, the quest dragon's 96 points of true damage as it is called is a bit too good. It one-shots every unit, the dragon can cast it twice per round and thus six times till anything can get close enough to hit her ... and I think it's female.
Thus, if you cannot take him out at a distance (which is also quite hard due to his also formidable defense), only four of your units survive. And if the dragon then also has some company ...?!
Well, you better bring your own dragon and fire first or be prepared to get fried alive. Or bring unit that can trap or confuse her and keep her from doing anything ... Going muzzle to muzzle with her is however suicide
Rabenhoff