Hearthpwn

Note: Wild Magic make this is a RNG-heavy fight. This deck won't work every time.

The basic idea is to have as much cheap hard removal as possible (including poisonous minions) to survive the early game. After that it becomes a classical "deal with the AI threat one at a time" heroic boss fatigue fight. He draws 2 cards when he turns into Nefarian on turn 1 so he will reach fatigue before you do and Power Word: Glory can single-handedly win you the fatigue race.

Don't overcommit on the board since he has Flamestrike.

Restart the game if he coins a Twilight Whelp on turn 1. It is not unmanageable but can screw your ability to curve.

As I said this is a RNG-heavy fight. Being a Priest mitigates this since most Priest spells are situational but you're never safe from a lethal-giving Mind Blast or a Dragonfire Potion which on top of that won't damage his minions most of the time. The random card you'll receive from Ragnaros also creates variance. Living Lava and Son of the Flame are the best because they can create a board swing while DIE, INSECT! and Whirling Ash are the worst.

Second try with this deck, not counting the game I restarded because of a coined Twilight Whelp.

More info on this fight : https://hearthstone.gamepedia.com/Lord_Victor_Nefarius