I can't seem to kill Diablo in Nightmare on my DH under any circumstance. Most of act 4 has been a cakewalk right up to him, but I just take so much unavoidable damage that I haven't even been able to push him into phase 2. I've been trying to use smoke screen, but you can only pop that like.. twice.. then you're out of discipline. Even Diablo's melee attacks are hurting like hell, despite my dh having 15k hp.
Even if I stocked up on vitality, I can't see how I'd get through phase 2 without the two healing wells there. Instead of a light breeze killing me, it'll just take a stiff wind. And Diablo's roughly hurricane force, or so it seems.