Is Vald that fellow you met in that cave full of spriggans or is he someone I haven't met?
Valdimar, your Housecarl from Morthal. He is awesome.
Oh, embarrassingly, I've never used Dawnstar and Morthal's housecarls, too lazy... When I find them I am already in a level in which I am so destructive that asking them to follow me equals blasting them with Soul Tear right away
But anyway, Valdimar begins combat with an alteration spell, so he might be durable enough to do the same thing as Talvas (the guy I mentioned before) and his conjurations, which is soaking damage and letting you attack the EW in peace. But he isn't essential, so be ready to step in in case he gets low. Also, EW is immune to Bend Will (I didn't know and died), so don't rely on that.
If you feel like hunting Karstaag down, the Standing Stone of the Ritual might be of use, with all the Rieklings you kill, and you can use the Aethereal Crown to keep that active stocked literally in your backpack ready for use. DO NOT use Mehrune's Razor to kill Karstaag. He is apparently immune. (After dying so many times I looked it up and it turned out Karstaag was one of the few who were immune to the dagger's effect -.-)
Anyway, if you don't feel like gathering all those items/people/buffs, you can always max your Conjuration, and make sure you keep him busy with atronachs long enough for you and Vald to chunk down his health (though he will heal back up fairly often). There is also a Troll and a Dragon nearby when you fast travel, and you can start combat right away to take advantage of their distraction. Or summon Durnehviir/Odahviing and use Storm Call (the Vigil is still outside, so why not taking advantage of this?). A hero of Sovngarde can also tank his attacks for a while. I don't know whether or not he is immune to disarm, but I think pickpocketing his gear is possible, so you can leave him completely naked before the duel!
Or FusRohDah him from the cliff! It will take a good portion of his HP away! Beast/Vampire form take him down especially easily too, but if you plan on staying human, at least empower yourself with Dragon form before combat, and use your racial power (histskin helped me when I ran out of potions). Anyway while he stands back up from a staggering blow or a shout, use your power attacks, don't waste stamina while he has his full defense, or he will just hit you as hard as you hit him, and render everything you did worthless (because he heals his damage back).
But some of these tips depend on how your character fights too... :/