This guy was insane. He was resistant to all three destruction magickas, elemental cloaks, and shouts. And he had some kind of defense against Arniel's Shade that made the shade punch. The room was full of dwemer spiders which could not be resurrected with the revenant spell. Later, my frost atronach did lots of punching damage, but couldn't punish faster than Orchendor could heal. In the end I equipped my mage with an ebony sword and went at him. My one-handed skill was so low that I leveled the skill four times in the dozen or so hits to make Orchendor Orchen-die. Was there a special way to kill him with magicka without resorting to weapons? Even Marked For Death, was no good on him I think.

Orchendor is the target of the Peyrite quest.

Try conjuring Dremora Lords

I don't think Mages are... at least, not without a way to deal non-magical damage.

Once again Dremora Lords

The funny thing is that a decent stealth character can kill him in one attack... I think Skyrim rewards the spellsword type of builds or mixed battlemage/conjurer type of characters.

Well, that answers that. Dremora Lords. My conjuration is 65, and here is something to know: I have never seen my conjuration level while I used Arniel's Shade.

Either it is bugged or was meant to be that way.

As far as I remember he is only immune to fire, frost, shock and shouts. Not stuff like:

1. Physical attacks from Dremora Lords or Frost Atronachs
2. Paralyze and then physical attacks
3. Poison

If I am not wrong, Weakness to Fire poisons can render him vulnerable to your fire spells.

Sanguine Rose would be a sure way to do it. It's magicially summons a hard hitting dremora that'll surely win the fight - maybe even take a companion as well so they 2 can take him on.

So you have to do one Daedric quest to complete another? What is this, Megaman?

Sure why not, you even get to have a drinking contest with a Daedra, what better way to start the day