The essence of the tactics when playing as the Dark Elves.

how to develop a Dark Mystic

Irbeth, perhaps? It's better not to play as her at all; she's the weakest hero in the Dungeon faction. Again, if the map is medium or small, the standard build is: Chaos + everything into magic. If the map is large, then there are two options: Chaos + Mentorship + a lot of Rogues, or Dark Magic (if there's an opportunity to acquire spells). Everything mentioned above is just my opinion.