How to kill Black Dragons?

I remember, I think, defeating a black dragon 1-on-1 with a Magic Dragon, but I'm not sure.

Hmm... was the hero a master tactician, decked out with artifacts that gave bonuses to troops? ...and wearing the Ring of Great Negation? Then, in principle, it's possible.
Otherwise, the best option is to find the aforementioned ring and cast some nasty spell on the dragon.