how do you dealing with black dragons as a mage ?

[YENKO]

Since dragons have such a high initiative, investing in Readiness after hitting Demonia may prove useful.
Also, buffing high-initiative stacks (like lake fairies+Anga's ring) will allow you to have the first strike. Not spells on the dragon itself, but something else like spawning extra fodder units and place them on the dragon path, like Demon Portal, Phoenix, Phantom, Trap etc.

In KB there is no such thing as 'universal army composition'.