[Stanis]

That's not an easy fight, you have to use the felhunter instead of the imp, use your shadow ward, use the spellstone and remember that generally , priests use their shadow resistance buff, which let them mitigate the damage from your dots and sometimes resist DC.
That mean, instead of SB try to use immolate/incinerate.
If it's an undead, use DC to heal yourself. If you don't get the message 'immune' try to fear...(they may use their racial thingy and if the fight is a long one, you may actually fear an undead once that fade)
And since they don't melee, you can use drain life/mana even without the talent to avoid being interrupted.
Those are some ideas, based on your build you can make your own strategy.

-Stanis