Get fire resistance gear.
As a warlock ∞
- Dot him as he's about to change, but be careful to not do it early, lest he whirlwind, dump aggro then chase after you.
- Searing Pain is your primary tool. I've switched between that and some shadow bolts just to help proc the bonus damage from crits. But that depends on if you have other locks or shadow priests with you.
- If you lose your pet, keep fighting and keep aggro. Resummon only if you can, and there are some good moments to do that. The fast-summon is very nice to have if you lose your pet.
I don't seem to ever get my inner demon, which is a good thing..