Hearthpwn

The goal with this deck is to avoid death and outgrind him.... Vaporize, Mirror Entity and Frost Nova + Doomsayer allow you to do this. Use your freeze effects and allow him to build up a huge board, then get massive value out of flamestrike. Once you've stabilized board control, heal up with Illuminator. You should be able to grind Nefarius down until he has no cards in hand and then it's basically GG. Use Kezan to steal secrets he gets using his hero power, bonus if you can Echo one.

I originally tried with Clockwork Giants, but they got stuck in my hand as I drew them after I'd grinded out his hand. You could also try Molten Giants, but I'd be hesitant because I really didn't drop too far on health.

Could probably take Antonidas out for something else, same with KT (originally for controlling board with giants). Antonidas will make for a good finisher.

Don't drop Lorewalker if you have any good spells you may need to use soon (like Flamestrike) but once you have board control and most of his cards are coming from hero power, it's pretty solid.

Mulligans: It's really important to have frost nova + doomsayer, or at least part of this combo. mad scientists are also amazing. It's really tough if you don't curve nicely into blizzard and flamestrike.

Edit 4/25: Took out a few cards that underperformed and added in some stuff to make the deck more consistent. It's a lame way to do it, but if you can drop Kel'Thuzad and Annoy-o-Tron on turn 10, you win. Annoy-o-Tron also isn't terrible early in the fight just to save some life. I took out polymorphs because they only handle one minion and to win this fight, you have to deal with multiple enemy minions at once. Coldlight Oracle is better than Arcane Intellect here because losing to fatigue is definitely a possibility. With coldlight, Nefarius will get there before you do.

Took out Kirin Tor Mage just because I felt the body wasn't really necessary. It definitely helps and can trade with stuff like Volcanic Drake, so put it back in if you'd like.