how do you dealing with black dragons as a mage ?

[Lathspell]

I managed it with the mass dragon slayer spell + elves and hunters. Faeries kinda suck against them, since they have magical damage. Also the ghost sword spell goes through their resistance, so that's an option as well. And of course, the Reaper's Black Hole is wonderful. Knights are another nice option, and their speed is not that big of an issue since the dragons will come to you. Couple that with the sword and the shield that give bonuses against them + dragon slayer spell, and you've got a killer stack. The nice part is that they're in infinite amounts at the castle by the time dragons pose a real problem.