You can use the "cmd" Skill in MythicsMobs to have mobs cast spells. You can check out an example skills config here if you like. A basic example would be:

Lightning: 
  Cooldown: 10 
  Skills: 
  - cmd 'castp <mob.uuid> lightning' >0 1

This will have the mob cast Lighting. The spell will be cast in whatever direction the mob is facing, as if the mob cast it.

The special "<mob.uuid>" in place of a player name tells magic to create a Mage for the mob's entity. This will make the mob function just like a Player casting spells. This means you can use Counterspell on it (it has an undo queue), you can cast Curse on it to prevent it from casting spells for a while. Its spell casts will show up on dynmap uniquely.

Automata will also react to them as if they were players, so Hunters will hunt them. Magical MythicMobs can add a lot of depth to your server, and provide a scaleable way to add "boss mobs" and other challenges for your powerful Mages.