@PolizPolice in most stories, and in D&D which most stories use as a basis, dragons have high resistances to magic to the point they can be practically immune to most spells. So it's not surprising that most magic spells can't beat it.
Also nobody knows at what point in their journey they fought the killing magic demon it may have happened towards the start or half way through. So they wouldn't have been at their peak at that point. Not to mention they had no way to defend against his Soul Track magic. It would go through literally any magic barriers and magic defence armour back then. It would be literally death if they got hit. That's why they sealed the demon, so they could come up with a countermeasure.