I swear it took me a good 15 to 20 minutes to bring this guy down, and even then it was with the help of good old fashion sword.

He's nearly immune to most magick I cast at him. Has a teleport spell that he uses a lot. Apparently has a near unlimited supply og magicka, and he can heal himself.

I'm sure as a rogue you could probably just backstab him and be done, but as a mage how have others dealt with this guy?

I found Morokei to be a much harder fight as a mage.

Really? I found him to be a push over. He's pretty easy to keep stunned if you have the Impact perk from the destruction tree. And with a Fire Atronach adding his damage to mine I was able to put him down pretty much without losing more then a bit of my health the one time I missed. I did have to use a bunch of magicka potions, but I had plenty so it was not a big deal.