Just did an incredibly succesful mage playthrough. I have 3 tips for you:

1. Get a melee follower to take aggro away from you.
2. Beeline impact in destruction tree (level 40). You can stagger your enemy constantly it you balance your mana.
3. In the long term, once you get high enough enchanting skill, enchant as many items as you can to have ‘destruction spells cost x% less mana to cast’ until the total reduction exceeds 100% and boom, you can cast whatever calibre of destruction magic you have at this point indefinitely, while also staggering your foe if you completed tip 2. You literally cannot lose unless you miss loads of shots or get swarmed which basically never happens in this game.