As a vampire? Drain life does terrible damage to Harkon as Serana is (I think) classified as a follower, melee does the same. He's immune to Corpse Curse. I have every Vampirism perk except Blood Healing. I have the Ring of the Erudite and Beast. Dont have the amulets. On top of that he ends up healing anything I do to him in the long run. I have level 100 illusion if it helps.

I killed him before on legendary with a dragonbone weapon dual wielded but this is far different, do I need to get the Amulet of the Gargoyle?

Anybody?

Do you have to fight Harkon as a VL? I don't think its possible.

You might just have to go undead on his ass.

No, you don't have to fight as a VL. But then using mayhem causes Serana to turn against me lol. I tried using Auriels Bow, but I epically failed.

Maybe I could spam an alteration of Calm and Auriels Bow? With a lot of soul gems

This is now turning into spoilers, but I've only used the Bow to take out his shield.

Lower. The. Difficulty.

I could try using fear + Serana gradually killing him and flames, and I'm an altmer so highborn helps

gonna be honest, I don't think Illusion will be of much help...

bring a large load of health and magika potions... if you have any conjuration spells use them... if you did the Darkbrotherhood I find the good ole Spirit of Lucien LaChance to help alot if you summon him bout half way through...

when he goes and activates his "shield" above the Alter, quickly hit him with an arrow from Auriels Bow, this breaks the shield almost instantly (and the Shield causes him to regain health, so the quicker you take it down the better)

(I have heard Sun Hallowed Arrows do extra damage on him, but have not seen any proof)..

If I'm lucky I can get him down to 3/4 HP before dying, I am yet to see him go and use the shield once,

Why do you HAVE to use Vampire lord form, why not just try to kill him as you would any other enemy? Or how about lowering the difficulty.

It's because he absorbs more health than the damage you can deal.