"Their defense seems to work by the dragon encasing itself within a strong energy shield that repels most magic and conventional attacks. While in this form they can siphon the unused magical energy wasted in basic spell casting, and then focus, amplify, and direct it back at the unfortunate caster in a sort of "feedback" attack. This ability is very powerful but requires a great deal of effort, and can only be used sporadically, requiring the dragon to rest between uses."
Either a spell redirect every X turns, tapping of opponents unused mana on a turn or using the extra damage of a spell cast on another minion that had more than was needed to kill it.
It will never happen but I am lore dorking today and thought it was an interesting idea.