So I give this information as both a guide, and as a warning. Playing the mage char is pretty tough. You get killed pretty easily. So I found a way (without cheating) to not get killed. But be warned! this makes the game REALLY easy, even on hardest difficulty.
Choose a Breton, with sign of the mage (or atronach) Specialize in conjuration, illusion, destruction, and sneak. And play through the mages guild to become archmage (I actually became immortal about 1/2 - 3/4 through the mages guild). Becoming a vampire helps.
Find some absorb magica, and resist magica items. My char (at level 10) already had 100% resist and 20% absorb (as mage, not atronach). Enchant these items yourself if you don't find them, using azura's star.
Practice summoning monsters. You can get them REALLY cheap if you get up to master level. Summoning a zombie only costs me 9 magica. I can cast zombies forever, as fast as they appear, just summon a new one, and NEVER run out of magicka. My current prefered summon is the wraith, but I might go find a xivali or atronach spell.
Buy an invisiblity spell, or become a vampire (but the vampire is only once a day, the spell I cast over and over. Vampire is usefull anyway for the skill and stat boost tho).
For me, the game now cojnsists of : Become invisible, run past all monsters, except those that I choose to kill because of loot or plot. When I choose to kill, summon a wraith. Either go invisible again, and let the wraith kill the mob (summoning and going invis again occasionally) or help out with 70+ point fireballs. I have 400+ magicka. I can cast 4-5 fireballs without using any magicka potions. Not many creatures can tank the fireballs and the wraith. This works for all the oblivion gates (where you dont have to fight at ALL, just run past and grab the sigil), all the dungeons, everywhere pretty much.
So far none of the creatures can see invis, or bother to cast detect life. I have found that zombies will occasionally see me while invis, but only if I have been fighting that particular zombie already, they will never find me as I walk past.