Guide for Heroic Nefarian! Skips the Onyxia part of the fight!
Win by killing Nefarian in one hit which skips the Onyxia phase. Build a big taunt with buffs and constant healing. You'll usually want to start building the big minion when Nefarian can't kill it with what's on the board plus an additional 4 damage from his removals. Often, this will happen just after you Holy Nova his bone constructs or if he starts with normal minions and can start a Deathlord with Divine Spirit and Velen's Chosen. Use Mind Control Tech and Dark Cultist early to ward off damage.
When you do have a big taunt established, playing Kel'Thuzad will stop the AI from attacking at all and you'll further be able to heal your big taunt. (Fixed in Karazhan patch) Use a big taunt with excessive healing/health pool or Kel'Thuzad with sacrificial minions to keep your board healthy until a one-hit-kill is possible. Either use Inner Fire on your big taunt or Lightwarden [/card](from [card]Light of the Naaru) to ready the final blow of this adventure.
IMPORTANT: Do not ever break Nefarian's armor unless it is a killing blow. Breaking his armor starts the Onyxia phase from which you will never be able to recover. For when Onyxia is defeated, Nefarian returns to the board and kills ALL of your minions. Keep Nefarian at around 1 digit's worth of armor since you don't want an accidental break from Holy Nova.
Mulligan for early minions such as Mind Control Tech and Dark Cultist. The early spells, Holy Smite and Power Word: Shield, are also worth keeping if you start with an early minion. Lightwell may be a low cost minion, but it's far more important to keep this safe in hand until it can heal from behind your big health minion. Do what you can to survive Nefarian's early minions. If he plays his hero power for Bone Constructs, you may be in trouble. It's worth using Holy Smite and trading early minions into these to survive until you can clear them all out with Holy Nova. If he plays minions from his hand, you will likely be able to play early minions safely and get a Deathlord or other high health minions with buffs started easily. Always use your healing on your big health minion; it's survival is what will win you the game later.
Continue to buff your big health minion. Save Holy Nova, if your taunt can afford it, for a turn when it will kill multiple Bone Constructs. Play Kel'Thuzad as soon as you are able to, ideally with smaller minions already on the board to trade into Bone Constructs every turn afterwards. Don't neglect to heal your big health minion when Kel'Thuzad is out. It's possible that Nefarian will kill your taunt or Kel'Thuzad with Shadowflame if you stop playing conservatively. Eventually, you'll want to get your big health minion well over 30 health and use Inner Fire to strike down Nefarian in one hit. Lightwarden can also be really powerful, but you'll have to buff its health as he'll probably kill it with lava or his other removal spells. Alexstrasza is also useful here to lower the damage you'll need for a one-hit-kill by 15, and will strike down multiple minions herself.
Since it's been a long time since Blackrock Mountain has come out (2 expansions and 2 adventures later), here are some options for other cards. Now that Naxx and GvG are also out of standard and must be crafted, there will also be ideas for replacements.
- Dark Cultist has fantastic value alongside Kel'Thuzad and even just for buffing Deathlord [/card]and[card]Lightwell. If you don't have Dark Cultist, Anubisath Sentinel or Spawn of N'Zoth can also trade into Bone Constructs while giving your other minions health.
- Velen's Chosen is an efficient health buff, giving 4 health at 3 mana. While it's a much more expensive card, Power Word: Tentacles is the same value, giving 6 health at 5 mana, the equivalent of Velen's Chosen and a Lesser Heal. (The spell damage rarely matters here)
- Light of the Naaru is a decent burst heal and provides a sometimes fantastic Lightwarden for the OTK. You can choose to run either Flash Heal/Darkshire Alchemist for the often necessary burst healing or Holy Champion for the extra OTK option.
- Deathlord is extremely important here, giving the best stats for a low cost of 3 mana. If you do not wish to craft one, there are a few possibilities. Try Mogu'shan Warden, Twilight Guardian (if you can fit in other dragons somehow), or Cyclopian Horror for weaker taunt cards. Or, try to use Sunfury Protector or Faceless Shambler on something for a more difficult to set up combo.
- Kel'Thuzad is no longer mandatory, but can still provide so much value in keeping Bone Constructs off the board (instead of passive in pre-Karazhan). If you must, I suppose you could run some other nigh-infinite value legendary like Hogger, Doom of Elwynn.
- Excavated Evil and Corrupted Seer are new board clears you could try.
- Brann Bronzebeard would get value here, if you have enough space.
- Justicar Trueheart (or even just Prophet Velen) could make healing the big taunt much easier.
- Fjola Lightbane and Eydis Darkbane have potential, gaining health from buffs while taking down minions for free.
This fight is very similar to the fight against Majordomo and Ragnaros from the second wing. Lots of strong minions will get spammed out by the boss and you'll need to be able to handle them accordingly. Fortunately, I think this fight isn't as bad. Holy Nova is already capable of handling the hero power minions without Velen's Chosen spell damage and Nefarian has fewer and more expensive removal cards than Majordomo. I even used almost the same exact deck as my other guide for Majordomo: http://www.hearthpwn.com/decks/226305-heroic-majordomo-otk-priest. How funny that two major rivals should fall to the same trick.
This is the final boss of Blackrock Mountain. Don't expect an easy fight. I was lucky enough to win on my second try, but there will still be times when RNG strikes down a good start. Good luck!