I split my mage between magicka and health.I took mana at every level.
And I put every point into destruction if available and conjuration if not.
I joined the magic guild as soon as possible, since some of the quest rewards have mana regen.
There is still a lot of running away involved with higher level mobs that have elemental resistance of any sort.
But in the end I found that being smart about your spells prevented most of the elemental resistance. I don't think any enemy in the game is resistant to all of the elements. I think the element the fewest are resistant to is lightning.
The really annoying thing with playing a mage is going up against other mages that use wards, because unless you use very powerful spells, they just get absorbed by the ward. What I found, though, is that as long as their ward is up they can only use one-handed spells, which are much weaker. So I could just absorb the little bit of damage they could throw at me and wait for them to run out of magicka on their ward, at which point I'd throw a couple of spells at 'em and they'd die.