Sets-Things-On-Fire will set you on fire for this offense!
Well, his (or hers) name certainly leaves little to imagination

Anyway, NPC mages are indeed extremely dangerous. You need to close in fast (sprint or Whirlwind Sprint) prevent them from casting spells (bashing, paralysis poisons/magic/powers, shouts) and beat them down before they can recover. Or use stealth and kill them before they see you. Also resist element and magic gear and potions are helpful.
For my current character the method is: hide, cast fury, watch the fireballs fly, repeat if necessary, finish off who's left, loot the bodies
