how do you dealing with black dragons as a mage ?

[Ahuitzotl]

Since they are not affected by magic they are tough. Fire resistant units are good vs. them, like Demons and of course your own Black Dragons.

The basic strategy is to attack them with some worthless unit to absorb their counter attack then pile on with everything else you have. For a mage the best way to create a worthless unit is the Phantom spell.