Ok, I'm playing on the default difficulty and manage to get Harkon down to just a sliver of health...but he keep using DL on Serana and any gargoyles I summon to heal all the way back up.
I have Auriel's Bow but the arrows don't do enough damage to overcome the rate of healing he gets from 2 handing DL spells.
Get full Dawnguard Armor and Dawnguard weapons and Melee him to death. Give Serana a Dawnguard weapon too... --- BETHESDA, just as quick as you are to advertise Dawnguard, I hope you guys are working on a patch so I can actually complete it...(Auriel's Bow)
Sunhallowed arrows make him Bat all over the place. Once he stops, melee the crap out of him. Proceed until he finally goes down for good. --- xbox gt: Doorstop Idol "You must be the change you wish to see in the world."
When he floats over the fountain of blood and is encircled by an energy sphere, you need to shoot him with the bow.
I never did that. I just drained more life out of him than he could Serena or my gargoyle. --- Laa shay'a waqui'n moutlaq bale kouloun moumkine. (Nothing is true, everything is permitted.) GT: EaglesNflFan36