Template:Nefarian (Hidden Laboratory) strategy
Nefarian's final appearance in the adventure is a particularly tough encounter.
The starting boss is Nefarian himself, equipped with the Bone Minions (Heroic) hero power. Once his Armor is removed, the action will immediately pause and Nefarian will rise into the air, with Onyxia stepping in as the enemy boss, equipped with 
Onyxiclaw, and using the Nefarian Strikes! hero power. This hero power is auto-cast, and will be used at the start of each turn. However, since Onyxia will appear immediately after Nefarian's Armor is removed, this will usually happen during the player's turn.
While Onyxia is alive, Nefarian Strikes! deals damage to the player at the start of each turn, with a different amount dealt each turn. The amount of damage done is: 1, 2, 1, 3, 1, 4, 0, 20 and then continue to do 20 every subsequent turn. Obviously, this makes defeating Onyxia fairly quickly a priority.
Defeating Onyxia will restore the original Nefarian boss, with his previous Health total, and will also see Nefarian immediately destroy all minions on the player's board. Once again, this takes place immediately upon defeating Onyxia, even if this is during the player's turn. Defeating Nefarian will complete the encounter.
Because of the different phases of the fight, intentional stalling can be effective in preventing the use of Nefarian Strikes! Players should also prepare for the board wipe following the defeat of Onyxia, and plan around it as much as possible. The player should generally try to lose as few minions as possible to the board clear; Deathrattle minions such as 
Nerubian Egg, 
Piloted Shredder and 
Harvest Golem can also be effective in recovering quickly. It should be noted that unlike the Kel'Thuzad boss fight, the player's turn does not end when a phase ends. Therefore, it can be wise to wait until Onyxia can be destroyed with the player having at least 8 mana still available, so that they can play Kel'Thuzad and immediately get their minions resurrected.
An alternate and very effective workaround for the encounter is to stall during the first Nefarian stage, and then perform a one turn kill (OTK), thus skipping the Onyxia phase completely, avoiding both the additional damage required to defeat the boss, and the subsequent wipe of the player's board. Priest is a good class for this, using a classic Divine Spirit/Inner Fire combo.
Normal
- Mage
The following deck can succeed against Nefarian, but will still require a lot of luck: 1 Mirror Image, 1 Secretkeeper, 1 Arcane Explosion, 1 Unstable Portal, 2 Mad Scientist, 1 Sorcerer's Apprentice, 1 Arcane Intellect, 1 Counterspell, 2 Duplicate, 1 Frost Nova, 2 Ice Barrier, 1 Ice Block, 2 Mirror Entity, 2 Vaporize, 1 Illuminator, 2 Kirin Tor Mage, 1 Fireball, 1 Polymorph, 2 Ethereal Arcanist, 1 Blizzard, 1 Dr. Boom, 1 Chromaggus, 1 Ragnaros the Firelord
You need to save the Polymorph for when your opponent plays their own Chromaggus. The Ice Block needs to come out as soon as possible, so as to allow as much use of your Ethereal Arcanists as possible. After you kill Onyxia, Nefarian will wipe your board so hit him for as much damage as possible when you break his Armor so he will be weaker when he lands again. Do not play any minions you don't have to before Nefarian lands, so try to kill Onyxia using as little as possible. All 3 of the legendaries in the deck may be needed to bring down Nefarian at the end, though it is possible to kill him with a very powerful Ethereal Arcanist. The legendaries will distract Nefarian from your other minions, such as the Arcanists, so as to allow you time to go for his face. The bosses will always go for your face unless you have a legendary or a minion with 6 Attack or more on the field.
- Deathrattle/Control
Several classes can use this strategy effectively; shaman is a good choice. The key strategy is to minimise the impact from Nefarian Strikes! and the board clear at the start of Stage 3, using Deathrattle minions like Nerubian Egg and Haunted Creeper, and high value control cards. Bloodlust Shaman decks work well for this strategy.
During the first phase, use removal such as Lightning Storm/Whirlwind to remove Bone Constructs and keep Nefarian's board down, while building a board of your own. Taunts like Defender of Argus and efficient trading options like Totemic Call and Flametongue Totem can be very effective. Take your time to establish a solid hold of the board, without concern for tempo. After a while Nefarian will start playing bigger cards such as Drakonid Crusher, which will be less of a threat since your Health should still be high at this point.
If you can manage to hold the board, Nefarian will quickly empty his hand, at which point you can push to Stage 2. During this stage you will suffer unavoidable hero damage, and so you'll want to complete this as quickly as possible. If you've built up a good board this should be easy, and you may even be able to achieve a one-turn kill on Onyxia. However, once Onyxia is defeated, your board will be destroyed. This is where minions like Harvest Golem, Piloted Shredder, etc, will come in handy, allowing you to spring straight back into action following the clear. However, be careful to lose as few minions as possible during this transition, by not placing any minions onto the board unless necessary.
Once Nefarian has returned, it should be a fairly easy match, so long as your Health has not dipped too low during the earlier stages. High value cards like Kel'Thuzad, Ragnaros the Firelord and Dr. Boom can be used to trade efficiently, and Nefarian should only be able to play 1 card each turn at this point, giving you a very good chance of overwhelming him with your larger hand and high value cards. With a few Deathrattle minions having survived Stage 2, you may be able to coast from here even at low Health, although be aware of his Soulfires and Tail Swipes.
- Priest
A simple and effective workaround for this encounter is to achieve a one turn kill (OTK), by using Divine Spirit and other Health buffs (Power Word: Shield, Velen's Chosen, buffs from Temple Enforcer/Dark Cultist) to build up a high-Health minion (Deathlord is ideal) and then use Inner Fire to increase its Attack to the same number. Once the minion is created, the player must defeat Nefarian in one hit, thus skipping the Onyxia phase and subsequent board wipe.
A OTK on Nefarian will require at least 31 Attack. Note that removing all of Nefarian's Armor without defeating him will immediately summon Onyxia, even if the player has not yet finished their turn; it's therefore vital to take him from at least 1 Armor to defeat with a single blow.
The main challenge of this approach is drawing the right cards, and preventing Nefarian from removing the key minion before it's ready. Nefarian lacks any hard removal, but it still makes sense not to play the key minion until you can afford to immediately buff its Health, allowing it to survive subsequent attacks - ideally have most or all required cards already in hand, then play several of them in one turn. Alternatively, focus on clearing Nefarian's board before playing the minion, and hope Soulfire/Shadowflame are not on hand to destroy it without direct attack.
Kel'Thuzad can be very effective for allowing other minions to trade repeatedly with Nefarian's, preventing them from whittling down the key minion. Additionally, a current bug means that with Kel'Thuzad and a Taunt minion on the board, Nefarian will not physically attack any of the player's minions, making this an extremely easy workaround for the encounter. However, Kel'Thuzad can still be removed with Soulfire, Shadowflame etc.
Heroic
A Mage deck can defeat heroic mode given enough attempts to get a good starting sequence. Round 3 or 4: Ice Block. Round 5: Sea Giant x 2. Round 6: Abomination to clear the enemy board. Round 7: Remove remaining armor with Sea Giants, Illuminator x 2. Round 8: Harrison Jones. Keep Onyxia up for up to the next 5 rounds to empty out the enemy hand, clear his board and fill your hand with 10 cards. The health restored from one or both Illuminator will counteract the hero power damage. Finish off Onyxia at the start of a turn before round 8 of the hero power damage, which would do 20 damage to you. Use all your mana to play as many minions as possible to replenish your board following the clear. The golden play is Archmage Antonidas against the empty enemy board. This will allow you to blast away every minion that the enemy plays and use the attack damage of Archmage Antonidas to burn down the enemy hero until he is in range to be finished off.
A Mill Rogue deck can handily beat Nefarian provided a few precautions are made. Very little to no unnecessary draw should be in the deck, such as Fan of Knives or Shiv, as the total 90 health of Nefarian will mean that every card counts (for this reason, Gang Up is essential). It should also be made sure that the boss is out of cards before entering Stage 2, as blowing through this stage with forced drawing and limiting Nefarian's damage as much as possible is necessary. Dark Iron Skulker, while not essential, is an extremely effective way of dealing with Nefarian's hero power. Beware however than Nefarian's Bone Constructs only cost 1 mana, and returning them to his hand is extremely inefficient.
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