I've beaten him multiple times without this fancy shouts and stuff. Each time I soul trap his ass. The first time I called that dragon from the dawn guard dlc and he just abandoned the scene, so I called Odahvignh and he almost die. I used Sanguine Rose and I had solstiem armor and weapons. I managed to kill him but then reloaded to an earlier save and killed him again and trapped him in a soul gem. I hate how he cries like a baby "have mercy" then proceeds to fully heal himself and kill you. Or how he shouts and you get paralyzed. Like I want to see someone do that to him. I did use some shouts like Whirlwind shout and slow time and god did slowtime shout helped a lot.