He is throwing Fire Stones at you(summoned prior to transformation; in Tenser's form he is unable to use any spells). To protect from that boost up your fire restistance and armor rating.
Should help.
Finally I got him thanks to "Invisibilty" and "Damage Health" bow poison, my character is Expert at Alchemy (my current level 27).
I kept shooting at him with empoisoned Ogre Arrows and drinking Invisibilty potions from time to time...
Thankfully he is no immune to Invisibilty like some OOO bosses, also he is not immune to paralysis.
If I hadn't used this tactic I couldn't stand more than 30 or 40 seconds faced to him.
brucevayne your magicka system is awesome, the best I have seen in Oblivion these last years.
You could make a link between your spells and Alchemy, especially ingredients.