How to kill Black Dragons?

by the way, snurov_hl, did you read post 120 in this thread? What do you think about this method? It seems to work very effectively... even for online play.

It definitely works.
By the way, I'll add one small detail. It's not bad to summon Phoenixes or Manticores first, and then cast Sanctuary. After that, the Druid hides in a corner and starts "re-summoning." The summoned creatures cannot attack. Around the third round, you can start attacking. :)

to be honest, I don't see any other ways besides having a more or less leveled-up hero.

Well, that's exactly what we're talking about - what kind of hero, and how leveled up. Of course, it's easiest with a Barbarian, with Immolation. But there are other options.
Vampirism on Devils works when playing against a decent tactician.
Teleporting Black Naga behind the enemy lines under Bless and Guardian Angels. This also requires some tactics, of course...
At the same time, skillfully move your heroes and the attacking unit so that the Dragons don't get the urge to switch to attacking the heroes. It's difficult, and it doesn't always work. :)