I was on master. My char was lvl 61 though. Didn't ever see him use anything but bats. I first thought to only use the bow cause of the "it's supposed to be uber against undead" aspect, that idea flopped with a resounding thud. Then tried going Lord to Lord with him, that ended significantly worse. Eventually I ended up, using my normal dragonbone gear (sword with paralyze 2sec enchant, he'd resist a lot, but it's made with a black gem and has like 138ish charges on it so I can swing it for days before it runs out and 2 secs may not seem like much, but it's enough to cause them to fall down and force them to go through the stand up animation, buying you an extra few seconds without the added cost) poisoned every swing (stam and magic poisons, he's immune to health poisons), and just used the terrain to hide from his shots then run him down swinging when he tried to get snippy and come out of running around. When I'd lash at him and land a few crits he'd kind of freak out and go bats trying to run away, then he'd run and suck health off serana and try to run me down, so I'd run and heal her till she retook aggro. Also used a Storm Atronoch since that helps depleat his magic and is a targeted ranged mob, so when I lost sight of him I just look where stormy's shootin, he's also a good aggro distraction. The key for me seemed to be keeping the atronoch up and using my main weapons instead of the new stuff from the expansion. Everything I made was like 3 times stronger than anything I found so not much use. I think he only did that bubble thing 3 times before he went down, and we killed him before he got it activated the third time. Also don't forget shouts, Unrelenting force doesn't do much (since he's flying) but fire breath put a boot in his balls (steel toed). Flame cloak is nice too, not sure it contributed much to the fight, but it was a nice additive when he was chasing me to close, still taking damage, but I can run and regen at the same time. Hope that helps.
