I hurled dual-cast Firebolts at it until it had about a quarter of it's health left, then realised it was fighting something else (I had it stunlocked most of the time so it didn't have much chance to attack).
Curious, I edged a bit closer and there was a mudcrab, stoically battling this fire-breathing leviathan.
Humoured, I stopped attacking the dragon, then lo and behold! The dragon rose up on it's hindquarters and it's keening death-wail ensued. The great beast toppled to the ground and the lowly mudcrab shuffled away with complete nonchalance.
Interestingly, the dragon was actually attacking this resilient crab, but was outmatched!
Talk about punching above your weight, what an upset!
As an aside, when I went to claim the dragon's soul the cocky little crab must have thought it could take me, being bolstered by it's huge win over the dragon and all. Sufice to say that about two seconds later it was cooked in it's own shell. A shame, really. Imagine the stories it could have told it's grandchildren . . .