OK, I am really puzzled how to do that in a smart way (lvl40 conjurer, vanilla game, PS3). Wiki says that a summoned dremora lord helps. Well, It took 4-5 summoned dremora lords to just bring him to the shadow form, and there are still 2 other shadows left. He also resists paralysis blade. Can wiki be that off? How would you defeat him, so that you wouldn't have a feeling of frustration after the battle?

What would be an optimal frost/fire resistance for this battle?

That was one of the most difficult fights for my mage on master. I had to give up first and return later with a follower from Solitude.

Bring resist-frost trinkets and potions. I think I started with a paralyze poison on him (bow), then conjured a dremora (sanguine rose), then used a scroll that set undead on fire and made them flee. After that a lot of running and desperation. (there was a very potent paralyze potion on a table on the right when you enter btw.).

Another approach: try luring the undead into the hallway and finish them there one after another unnoticed. It worked on an earlier playthrough on lower difficulty.

Mass paralysis on him and everything in the room. Run in to the back and stand behind the plinth with dawnbraeker in it to block his spell casting, cast mass paralysis either normally of from a scroll and procede to kick ass. Rinse and repeat as necessary.

Standing behind the altar was a good idea. Shadows are having trouble reaching there, so I could observe a little. So, his frost storm spell, actually, instakills a dremora lord. I think, it's a bit unfair, since the most powerful conjuration spell occurs to be so much weaker than his destruction spell. But that spot can really save life of a conjurer or an archer.

PS. The only reason he can be killed by summoned creatures/companions is because he is too busy reaching the main character.

As a mage I just juiced him with chain lightning, single cast in each hand (not dual cast). It means I can cast every second and hit every target in range AND take his mana down some. If you have impact, dual cast him with chain lighting in between mass paralysis as required.

Last time I was there, I just sneaked up to the entrance and blasted him with a bunch of incinerate. He was dead before he knew I was there. More incinerate to kill the shade and all the other shades.

Difficulty of the battle depends a lot on your current level and your build. A lot. Everyone has different experience with different dungeons.

It's a good idea to get as much magic/fire/frost/shock protection as you can. My current build has 40% magic resistance 80% spell absorption and 85% fire/frost/shock resistance.

Use the staff of Magnus.