That One Boss / Hearthstone: Heroes of Warcraft

"Join me in the darkness..."

— The Lich King, after killing you.

Hearthstone's single-player content is generally designed to be simple so as you don't get too confused. That doesn't mean it's easy.

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Adventures

• Curse of Naxxramas:
  • Sapphiron's Hero Power destroys all enemy minions that aren't Frozen, every turn, for free, forever. Normal Mode features a helpful minion who protects your minions from Sapphiron's icy breath, but on Heroic Mode? Nothing. Everything you play will die the second you press "End Turn", while Sapphiron gets to build his board without opposition. His signature spell, Pure Cold, is a powerful 5 mana spell that deals a whopping 8 damage to your hero, freezing them in the process - not only is it painful, but it also means that weapons won't be as effective in getting the kill. This fight is generally considered the most difficult part of Heroic Naxxramas, as it mandates extremely creative deckbuilding to pull out a win, especially since Kel'Thuzad has the balls to disable Alexstrasza's effect, meaning you need some serious firepower to whittle down Sapphiron's massive health pool.
  • The Paladin class challenge. Class challenges are rematches against specific bosses where the player is given a pre-built deck to fight them with, usually to promote fun strategies for casual play. There were two problems with the Paladin challenge, however. The first is that the deck you were given was absolutely atrocious, relying on giving huge buffs to the Echoing Ooze card, despite the fact that the deck had very few buffs in the first place. The other problem was the fact that the boss in question was Kel'Thuzad, the Final Boss of the adventure and its greatest challenge outside of Heroic mode. Kel'Thuzad boasts very powerful minions, has access to the Twisting Nether spell to nuke the board whenever he pleases, and has a starting hero power that puts the player on a very short clock to kill him. The fight was universally detested right from launch, and was so psychotically hard that not even top Legend ranked players could beat it in one go. The deck used to fight Kel'Thuzad was later reworked to make the fight substantially easier, but even to this day the initial incarnation of the fight is considered one of the hardest bosses in the game, more than even some Heroic battles.
• Blackrock Mountain:
  • Chromaggus. At the end of his turn, Chromaggus will put a Brood Affliction card into the player's hand, which give Chromaggus bonuses while they're in hand, varying from healing him to reducing the costs of his cards. Players can cast the Afflictions for a small mana cost to get them out of their hand, but Chromaggus will also play Chromatic Dragonkins, 2 mana 2/3 minions that gain +2/+2 whenever the player casts a spell - which, incidentally, includes the Afflictions. Finally, in the lategame, if the initial onslaught is survived, Chromaggus will be able to drop Nozdormu. Normally, Nozdormu is a joke card who reduces the turn timer for both players to 15 seconds. When used by an adventure AI, however, Nozdormu will glitch out and straight up skip the player's turn. Forever.
  • Heroic Maloriak, who is nigh-universally agreed upon to be the hardest boss in the entire game. Even with years upon years of Power Creep, this fight will beat your ass like a drum. Maloriak's Hero Power passively swaps the attack and health of all minions, making it impossible to play important minions like Doomsayer, while also giving his own minions +2/+2. This gives him a nigh-unbeatable stat advantage for the early parts of the game, while screwing with the player's gameplan. His trump card is Release The Aberrations!, a 2 mana spell that summons 3 1/1 minions with Charge - on Normal mode, it's no big deal, but on Heroic, this combined with his Hero Power equals a third of the player's health down the drain. To this very day, the generally accepted response to a turn 1 Coin into Aberrations from Maloriak is to restart the fight until he opens with something else.
• League of Explorers:
  • The Minecart ride. Unlike the other bosses, the player is given a pre-made deck to fend off an army of troggs for 10 turns until the battle automatically ends. The deck is unfortunately very prone to bad draws, meaning that the entire fight is ultimately a complete gamble to win.
  • The first battle with Rafaam can be this, depending on the deck you play. The boss steals your deck, forcing you to play with a (semi-bad) pre-made one. Oh, and the boss has Unstable Portal as a hero power. Good luck with playing against your own meta-breaking deck. Of course, you could intentionally make a bad deck with horrible cards, but even then Unstable Portal means there's still the danger of Rafaam getting something that your crappy deck simply can't answer.
• One Night in Karazhan:
  • The Crone. Her fight's gimmick gives out some good news and bad news: the good news is that Dorothee can give your minions Taunt or Charge depending on where you place them; the bad news is that this is an Escort Mission with Dorothee at only 10 hit points as your escortee; if she dies, the Crone's Hero Power kicks in and gives you what's essentially a One-Hit Kill to your hero. Unfortunately, she has plenty of resources to whittle down Dorothee and your Taunt minions from Stealth minions, her exclusive Flying Monkeys with Charge and attack power that can easily land a killing blow to Dorothee, the dreaded Knife Juggler, and even Twisting Nether that destroys all minions (including Dorothee) on the field. Lord help you if you're playing on Heroic Mode, as on turn eight the Crone automatically draws Twisting Nether; have fun trying to deal 60 damage to her in eight turns.
  • The Warlock class challenge pits you against Julianne with just about the worst deck possible for the fight. The deck you get is, in theory, based around getting an early board advantage and then winning off that; however, half the cards make you discard and the other half damage your hero, meaning that you'll run out of steam very quickly. Meanwhile, Julianne has the 4/2 Romulo swinging at your face from the start, and he must be removed in order to damage Julianne. Given that her deck is 90% Taunts and 10% removal, this is much easier said than done, and she can easily kill you before you can do any sort of meaningful damage. To add insult to injury, the card you get for beating the challenge isn't even very good.
  • Heroic Chess is a very demanding Puzzle Boss where you need good positioning and good draws to push enough damage for lethal before you run out of pieces, and since you use a preconstructed deck and hero, your collection has no bearing on the difficulty of this fight. While on Normal you could just Discover more pieces with your Hero Power to win through attrition, that isn't an option on Heroic; your Hero Power is only good for repositioning pieces, making the Black King's Hero Power to destroy your left-most piece much more devastating. To top it off, this encounter only became harder with time; while earlier versions of the fight had the Black King only start using his Hero Power on later turns, he is now much more willing to use it early on, which can drain enough pieces that the fight becomes unwinnable.
• Knights of the Frozen Throne:
  • Lord Marrowgar. His hero power heals him to full HP at the end of each turn, so no amount of face damage matters unless you have enough to OTK him. Meanwhile, he'll make your life as tough as possible by dropping hefty minions to wear down your board and your life, scything down minions with his Windfury weapon and Bonestorm spell, and most infamously, summoning 8 health Bone Spikes that deal 15 damage a turn if not removed immediately. It often takes just as much luck as it does skill to edge out a win. And did we mention that he's the first boss in the adventure?
  • The Lich King is an exceptionally tough customer even by Final Boss standards. He will open every fight with a special spell that changes depending on what class you're using to make the fight more challenging. While Priests are simply afflicted with a joke spell that muffles their emotes, giving an easy way out if you just want the free pack, the rest have to deal with debilitating effects that make the fight unwinnable without a very specific deck, ranging from giving the Lich King 100 armor to setting your Hero's health to 1. Even with a proper strategy, he does not mess around, spamming the board with plenty of tokens to buff with Anti-Magic Shell and Bonemare , abusing his massive health total by Obliterating your stuff left and right, and packing enough Freeze effects to make your average Mage jealous. On turn 7, he draws and plays Frostmourne, a 5/3 weapon that can't break, makes the Lich King Immune, and replaces his board with 6 2/6 minions (though at the very least, he can't play cards while Frostmourne is active, not that he needs to). You can only break Frostmourne by removing the 2/6 minions, but once you do, whatever board he had beforehand is summoned back, and he upgrades his hero power to one that deals one damage to your hero, increasing by one every turn, meaning that the game has to be finished fast before the damage becomes overwhelming, especially as his second phase is very good at tearing through your health and will likely leave you with single-digit HP. If you want the Arthas Hero skin, you have to beat him once with every class. Good luck.

Dungeon Run:

• Candlebeard, when fought late into the run, gains a hero power that gives any minion he plays Charge - and his minions have quite a bit of attack to make use of it. An unlucky player who didn't draft enough taunts (or plays a fragile class like Rogue or Hunter) is probably going to be forced to reset, because there's not much that can be done against his sheer aggression.
• Seriona is an Early-Bird Boss; she runs a Dragon Priest deck, which allows her to make use of extremely powerful cards such as Drakonid Operative and Duskbreaker, the latter of which is prone to annihilating the player's aggressive momentum. Additionally, she possesses two copies of Flash Heal, making her more durable than she appears to be. An unsuspecting player might use their damage cards on her in an effort to blitz her down, only to be undone in a flash. Furthermore, if the fight drags on for long enough, she starts using Sleepy Dragon, a fearsome 4/12 Taunt that can cause the player to lose so much momentum that they've practically lost.
• The Trapped Room is a headache no matter what class you choose. It starts the battle with five random Secrets loaded onto the board, and should any of the Secrets be activated, it will summon a 3/3 Buzzsaw and then plant down more Secrets when its turn rolls around. Logically, the answer should to be plain as day — do nothing. The issue is that the Trapped Room also has access to Ethereal Arcanist, which can easily snowball if not dealt with, Secretkeeper, which gets more powerful as the Room drops more Secrets, and Shroomsayer, a 4 mana 7/7 with a self-destructive effect that also tempts the player to do something to answer it. Essentially you have multiple options and they're all terrible — do nothing and get bashed in by Arcanist, or attempt to deal with its few minions and inevitably spring its Secrets which lets its Secretkeeper grow stronger. The worst part is when its active Secrets have overlapping triggers, which leads to a single action causing a cascade of responses and at least two or three Buzzsaws popping up. Your best bet is to pray it doesn't draw any of its big threats, or that its active Secrets don't trigger on your few answers to them.
• Blackseed continuously plays minions with "summon another minion" battlecries and then buffs them with his hero power, which transforms them into a random minion whose cost is one mana (early encounter) or three mana (late encounter) higher. His side of the board can quickly snowball out of control unless you've picked up plenty of removal and board clear options during your run.
• Waxmancer Sturmi easily becomes the nightmare of any Deathrattle-based run, as her Hero Power makes a 1/1 copy of any minion on the board. At later levels, it instead straight-up copies the target. Naturally, her own deck is filled to the brim with potent Deathrattles, and yes, she will copy your minions too if they have good effects. If she gets the dreaded Sylvanas and starts copying her, you're now fighting an uphill battle as she takes your minions. This is also the one fight where playing the normally game-breaking Wax Rager is outright discouraged.
• Voodoomaster Vex is no slouch either. Sure, if your deck is loaded with powerful battlecry and deathrattle cards, his hero power can benefit you, but if he starts pulling out things like Aya Blackpaw plus Carnivorous Cube, you're in deep trouble.
• Spiritspeaker Azun is brutal if you don't have a Deathrattle-heavy draft. Because his deck is based around the Awaken the Makers quest, failing to kill him quickly means he'll drop Amara and heal himself up to 40 HP; since he appears at around encounter #4, there's no way in hell to kill him before dying to fatigue at that point. However, rushing him down is difficult at best, as he has two Mistress of Mixtures which, thanks to his hero power, heal him for 8 health each; if he gets Spiritsinger Umbra on the board, that heal goes up to 16. And that's before considering that he has N'zoth to push for game and cards that let him copy stuff from your deck, including those Game-Breaker treasures you picked up.
• Azari the Devourer is easily the most feared of the five potential final bosses. His actual deck is pretty straightforward, mostly consisting of board clears and demons. His Hero Power, however, destroys the top two cards of your deck, for free, every turn. Against the other bosses it's just a matter of holding fast until you can draw your win conditions, but Azari forces you to close the game out quickly. If you can't, than you have to hope against all hope that you can draw your game winners before Azari eats them. The best bet is to try and rush down Azari before you succumb to inevitable fatigue damage, but he also sports some very annoying Taunt minions to slow down your progress.
• Think Azari's bad? King Togwaggle has something to say about that. His deck is themed around kobolds, which seems pretty weak at first. His Hero Power, on the other hand? Gives him a random Dungeon Run treasure, like the overpowered ones you've collected thus far. Minion-heavy board? He'll use Wand of Disintegration, removing it. Alternatively, he'll use THE CANDLE, a one-mana Flamestrike that adds a copy to his deck. What's worse is he'll never run out of treasure. In addition to the above, Rakinishu will intervene every so often and deal 4 damage to you or one of your minions. All you can do is hope to draw your win condition before Togwaggle and his treasure overwhelms you.

Monster Hunt

• Inquisitor Hav'nixx packs a 4-mana hero power that halves your HP. He also uses cheap removal like Shadow Word: Pain and Shadow Word: Death to stop you from rushing him down, along with direct face damage from the likes of Shadowbomber and Mind Blast to finish you off. And if he gets Maiden of the Lake on the board and you can't remove her, Yogg help you.
• Cragtorr runs a deck based around the Jungle Giants quest and has a hero power that reduces the cost of all minions in his hand by 1. Combined with his various mana ramping cards, he can easily start spamming 8/8 minions from turn 4 or earlier, letting him complete the quest very quickly and start flooding the board with gigantic minions. Even if you manage to clear the board, he'll just use Nourish and Witchwood's Touch to draw up a new hand and keep churning out giant minions. To beat him, you'll need to either blitz him down in about 3 turns or pack an insane amount of board clear; good luck with either of those.
• Azalina Soulthief can become this if left unchecked. 3 1/1 wisps hero power? 4-mana destroy all wisps and gain control of a minion for each destroyed? Several spells and minions that buff these wisps? Yeah, have fun. Screw This, I'm Outta Here!
• The Face Collector uses an exact copy of the player's deck, excluding passive effects, i.e. he can use all the game-breaking crap that you can. This means that no amount of preparation or forethought can save you - you either need to get better draws than him, or make better plays. On top of this, his hero power is similar to Blackseed, but is much more powerful because it costs 0, meaning he can start snowballing immediately. If his board isn't kept clear at all times, he can easily end up with a 10-mana minion on turn 4.
• The Gnomenapper has earned infamy for just one card — Sack of Gnomes, which fills its side of the board with random Gnomes for only 4 mana. Individually those gnomes may not be really strong, but the sheer numbers will make it difficult to retake the board. Sometimes he even lucks out and pulls Rin, the First Disciple on you. Unless you've got a boardwipe ready when Sack of Gnomes happens, you're going to have a lot of trouble keeping up.
• Like the Gnomenapper, Experiment 3C quickly became The Dreaded for its Amalgamate spell. This card is basically Deathwing on steroids, destroying everything on the board and summoning a humongous token with the combined stats of all minions destroyed for 8 mana, which all but demands hard removal to get rid of. And it has three copies of this card in its deck, likely forcing you to save all your removal cards to deal with them. Which in turn makes you fair game for the rest of its extremely aggressive zoo deck, especially as 3C can easily refuel using its hero power. And just when you think you're in the clear? Say hello to the actual Deathwing! To summarize, this guy has a strong Zerg Rush strategy backed by insane amounts of card draw and the ability to clear the board up to four times while leaving a huge threat on the board each time; you will run out of steam before it does.
• Griselda can be a fairly tough Early-Bird Boss. Her hero power turns a random minion into a frog, so the fight is ultimately just an RNG-fest where you have to hope that she doesn't hex all of your minions before they get to attack. However, even if you do get lucky and Griselda hexes her own minions, the frogs are still Taunts that slow your progress, and she packs a ton of damage spells that can simply grind any offense to a halt and stall until you die to fatigue.
• The Mute can be monstrously difficult if you don't have strong removals handy at all times. She plays Silence Priest, which is normally a mere novelty (albeit strong) deck due to the limited number of Silences in the game, but The Mute's hero ability ensures that she can always silence: Silence your Taunt and Deathrattle minions to shut them down, or silence the many overstatted minions she has whose massive drawbacks are no longer an issue. Have fun dealing with a 4/5 minion on turn 3!
• The Nemesis battles are specifically designed to counter the basic strategy of their respective heroes, and thus by default they are a pain in the ass. Example:
  • Infinite Toki, the Time-Tinker's nemesis. Her deck is themed around the Mage quest, which already makes her hard to deal with. In addition, her Hero Power gives a minion Mega-Windfury - that is, they attack four times a turn for two mana. The minion dies at the end of the turn, but that's the only reprieve. Been unlucky with the RNG? Infinite Toki will end your run. Oh, and if she reaches ten mana? She'll use Alternate Timeline, a 10 mana spell that restarts the game but keeps your health the same.
  • Glinda Crowskin, Shaw's nemesis. Her passive hero power is simple: She is immune as long as she has a minion. The problem? She has a million ways to keep her board full. Between spells that give her a lot of imps in one go, hideously sticky minion like Voidlord, and finally, her unique minion Crowskin Faithful, which is horribly undercosted and cannot be rid of permanently, and she can use Crowskin Pact to flat-out steal your minion and give her another Cultist to torment you with. The big cherry on top is that if she manages to get Blood Imp via her Kabal Trafficker, the fight is unwinnable. To be, uh, "fair", Glinda could become manageable if the player has a lot of silences and AoE, but guess how often you may run into those cards in Shaw's pool?
• The Final Boss of the Witchwood, Hagatha. You start with 30 health, she starts with 100. Bewitch adds a random Shaman spell to her hand when she plays a minion; considering how long the fight is, you will be in for an arduous battle. Oh, and remember the Amalgamate spell that 3C had? She has it as well. You start with 3 overpowered Hero cards that add a passive effect, hero power and more cards to your deck when played - trust me, you'll need it.

Rumble Run

Unlike previous adventures, the Rumble Run's bosses use the default hero power for their class, and they only use cards and shrines available to the player. That doesn't make them a pushover, though; later bosses will have much higher health than you and their decks are always perfectly tailored around their shrine, while the player is at the mercy of the Random Number God. Some of these are, of course, harder to deal with than others.

• Wardruid Loti with Gonk's Armaments is known to give players fits. This shrine refills all of its controller's mana crystals whenever they gain armor; not only does this synergize with Druid's plethora of armor-gain cards, it basically turns the hero power into a "get all your mana back" button. It's also not limited to once per turn, meaning that in the late game, Wardruid Loti can easily give herself 30 mana turns. A free Ultimate Infestation (or, God forbid, two in the same turn) is never a pretty sight, especially since it'll likely be accompanied by a buttload of massive beaters that you likely won't be able to deal with since Rumble Run is exceptionally stingy with removal.

The Great Dalaran Heist

• Vas'no, a lategame encounter against a troll packing an Overload heavy Shaman deck and a Hero Power that causes all of his spells with Overload to cast again. This enables disgusting tempo turns where he can just flood the board with Spirit Wolves or Toads, or annihilate your field with Lightning Storm. While this usually forces him to skip a turn, he can also play Lava Shock or Eternal Sentinel to rid himself of the Overload and go to town again next turn. Heroic mode simply makes this even harder, as his spells cast thrice, meaning he can gain a board full of 2/3 Wolves as early as turn one, which is nigh-unbeatable without a particularly lucky hand. Tellingly, when Dalaran Heist was given a quick re-balancing, this was the only non-Heroic boss to be nerfed.
• Heroic Anarii Duskgrove is an infamous headache that happens to run a Token Druid deck based around Treants, with a Hero Power that summons two 2/2 Treants with Taunt. The sheer amount of pressure she pumps out every single turn is extremely difficult to keep up with, as clearing her board while developing your own is an exercise in fruitlessness. In addition, the sheer spam of Treants means that all of the crappy, situational Druid cards based around them (Mulchmuncher, Treespeaker, Dendrologist) become monstrously powerful, allowing her to secure her lead and go for the kill - all while dripping with total contempt for you. When nerfs were handed out to several Heroic bosses, Anarii got hit much harder than the rest of them, as her Hero Power had its cost bumped from 2 to 3.
• Bookmaster Bae Chao is an early encounter based around Silencing minions (already a Scrappy Mechanic). On normal she's not too bad since she only silences one minion at a time, but her Heroic version is an absolute nightmare. Her Hero Power is passive, and Silences every minion after it's played. This instantly shuts down every single Deathrattle, aura, self-buffing Battlecry, and so many other effects in the game, for no mana. If you don't have a deck of well-statted beatsticks and good spells, it's game over. Oh, and the boss herself is no slouch about her power either, packing every "Can't attack" and negative ability in the game, alongside cards to lower your minions' stats.
• Heroic Haro Setting-Sun made his sheer nastiness clear right out of the box with his Hero Power, which gave all your minions -2 attacks. Anything with one or two attack became functionally useless and anything with more than that still took a massive hit to their trading potential. Then there was his deck, a downright mean Resurrect Priest deck featuring heavy hitters like Obsidian Statue and Ragnaros, and because of how little damage you would be doing, it was extremely hard to kill him before he could stabilize. This fight was so thoroughly awful that Haro was nerfed to the point where he might as well have been deleted from the game - his deck was nerfed twice, his Hero Power now only reduces your attack by 1, and to top it all off, his encounter rate was slashed, making it hard to find him in the first place.
• Kazamon Steelskin isn’t a huge issue for most builds. After all, his hero power just prevents him from hurting himself with his own card powers. This becomes hell, however, if you chose to run a bomb warrior deck, which is pretty easy to do with the amount of synergy you can get from card picks, starting deck and hero power options. Due to how the bombs are programmed, they count as his cards for the sake of his hero power, completely negating your main strategy and relying solely on your minions.
• The final encounters with the Archmages (and Norroa) make for a fairly frustrating gauntlet. Norroa is the only boss with a turn-limit timer, on top of running a Secret Mage deck with every secret that exists. Vargoth spams the arcane spells gained from his Hero Power to destroy your board. Kalecgos ramps up spell damage with his Hero Power to end the fight in one turn if you can't rush him down (especially on Heroic, where it costs 1 as opposed to 3). And the elementals Khadgar summons with his Hero Power copy the spells he plays; resulting in insane spell spam that can turn the game around instantly.

Tombs of Terror

• Dire Bat is a total nightmare of an encounter, especially one that can be the second boss you fight. Its hero power prevents you from sticking any minion with less than three health on the board. It also packs Hunter's Mark, which means you can't sit back on big minions either. Poking it down won't work since it heals for 5 constantly - you need to take the board and kill it fast. Not to mention, the boss has been known to Hunter's Mark and kill off its own Sylvanas to steal your best minion and win the game from there.
• Until he was patched, Setesh suffered from a nasty bug that made him damn near unbeatable. While his deck was rather weak, his Hero Power can plant a debuff that causes a minion's owner to take damage at the end of every turn. Easily circumvented by trading minions, but due to a glitch, the debuff would be retained even if the minion died. This meant that the second you played a minion, Setesh needed to die, lest the stacking damage kill you in a matter of turns - a feat that was unfeasible without a hyper aggressive deck.
• General Husam has a hero power that gives all his minions Rush. You know, the same effect granted by Rocket Backpacks, one of the best treasures in the game which makes board control a cinch. As you might expect, keeping a minion on the board for a grand total of one turn against this guy is virtually impossible, and you'll likely fold to his unrelenting aggression.
• Xatma, Plague Lord of Death is easily the most feared out of the four Plague Lords. All of his Hero Powers are extremely synergistic with his Deathrattle-centric deck: Foreboding Fate allows him to capitalize on extremely high value Deathrattles like Sylvanas or Hadronox, which in turn protects the minions and allows him to trigger their Deathrattle effects again with his Hero Power next turn. Death's Embrace turns the aforementioned high-value Deathrattle minions into even stickier engines and triggers their Deathrattle effects again. Finally, Death's Gaze allows Xatma to completely wreck any token-based synergy (the strategy that the player most likely had employed to defeat his first two stages) by generating board clear after board clear while whittling down your Health.

Book of Heroes

• Jaina:Even for a final boss in an adventure with fixed decks, Aethas Sunreaver is brutal. His hero power deals 2 damage to your face for 2 mana and gives him Spell Damage +2 for a turn; while the second part is mostly irrelevant given that he's using an aggressive Secret Mage deck, it can still give his Fireballs enough damage for a surprise lethal, and since Jaina's deck has no healing (or even any defensive cards) whatsoever, every point of health counts. Unfortunately, this also puts you on the clock to try and kill him before he grinds you down... and Jaina's deck is horrifically prone to bricking with the sheer number of high-cost cards in it, and by the time you can start actually dropping your big cards, Aethas will probably have the Secrets he needs to counter him, wasting your turns and mana, and Jaina has almost no options for playing around his Secrets. And Aethas has 50 health while you start with 30. Your only lifeline is a hero power that damages two of his minions and Freezes them; you will need it, since letting his minions go unchecked for even a single turn will get you killed.
• Rexxar: Remember how, in the bonus campaign of Warcraft III, Daelin Proudmoore was kind of a pushover if you've been gearing up your heroes to turn them into a four-man army? Well, the Daelin Proudmoore fight in Hearthstone is pretty much the exact opposite of that. His hero power costs a hefty 4 mana, but it summons a 2/6 Kul Tiran Warship. These things must be removed every single turn, because if one lives to the start of his next turn, it pulls two free minions right out of his deck. This can easily cause his board to cascade out of control and overwhelm you with sheer weight of numbers... but even just getting your board set up to contest the first few Warships is a problem since he has more than enough cheap minions and weapons (such as Stormforged Axe and Kul Tiran Chaplain) to clean up your early minions and Secrets before they can be put to use. You do get a Hero Power that lets you deal 4 damage to an enemy minion, but since this obviously isn't enough to kill a Warship on its own, you're going to need an established board in order to do anything... and even if you do get your board set up, Daelin has the exclusive Monsoon spell, which will wipe your entire board with extreme prejudice.
• Uther: The Dark Portal is a pain to battle against. For starters, it has 100 Health compared to your 30. While you do have a Hero Power that deals 10 damage to it, you need a minion on the field, which isn't an easy task because of the Dark Portal's deck type. It plays an aggressive Zoolock and has a board buffing Hero Power, which can easily allow it to rush you down before you get any time to react. Stack this on top of the fact that you're using Holy Wrath Paladin against it (which is a rather slow deck and that you only deal a quarter of your opponent's health instead of nearly all of it), and it can easily steamroll past your minions and destroy you. The main saving grace is that you have a buffed Turalyon on your side (that costs 6 instead of 8) that can deal with the buffed demons. Other than that, you need quite a bit of luck to beat it.
• Garrosh: The Anduin Wrynn fight is very frustrating to get through since he runs a Divine Spirit - Inner Fire combo deck, which mean that any minion you left alive can be buffed up to deal lethal damage to you very quickly. The deck you were given this fight has a very limited amount of removal and no weapon, relying instead on Rush minions, which Anduin can easily dealt with his amount of low cost removals.
• Valeera:
  • Garona Halforcen is this book's version of the Dark Portal. Having 50 Health might not seems as powerful, but she made up for her lack of tankiness by her insane burst damage with Kingslayer that double any damage she deal to you. She starts the game with a Prep-Eviscerate combo that took a away over a third of your health, leaving you with much less time. The fight devolve into face race and praying that your random generation create some defensive or offensive measure to keep up with her until you can draw Jaina to create a Water Elemental. If that is not difficult enough, Garona pack Sap (to forcibly remove any Sleepy Dragon generated by your Steel Dancer) and Assassin's Training to draw her entire deck and discount any spell drawn by 1 to use her lethal to finish you off.
  • Jorach Ravenholt is the grand boss of the book and he did not disappoint. He pack 4 Permanently Stealth Shadows that deal 1 damage to all of your character (which counter a majority of your deck of low-cost 1 Health Stealth minion). One of the Shadow (Tess Greymane) pack a meaty 6 attack to hit your face with to make sure you pay dearly if you're hoping to kill her with just your Hero Power. The other three Shadows (Garona Halforcen, Taoshi, Fleet Admiral Tethys) lack in offensive presence is made up for with their high base Health, ensuring that you will have to use your Blade Flurry to kill them before they wipe out your Health.
• Gul'dan: The Tomb Guardian is the final boss of the book and can be a real hair-puller. Both you and the Guardian start at 10 mana, and you get a respawning 7/7 Cho'gall on your board. That's where the good news ends, because the Tomb Guardian has 100 health and a hero power that gradually destroys your mana crystals over time, forcing you to try and rush it down as quickly as possible with your biggest cards before you run out of mana - easier said than done, with the number of big Taunts and board clears in the Tomb Guardian's deck and Gul'dan's deck being full of weak minions and removal that's not nearly strong enough to deal with the Guardian's big minions. Its health total also means the Tomb Guardian is guaranteed to draw into Twisting Nether and DOOM! at some point to nuke your board, making it almost impossible to come back with your depleted mana barring extreme luck. To make things worse, Cho'gall has a Spellburst that makes your next spell in the turn cost health instead of mana, which is supposed to be a lifeline for you to come back from having your mana drained, but it's bugged and doesn't actually work.

Book of Mercenaries:

• Tamsin: Bolvar Fordragon. Good god, Bolvar Fordragon. Every turn he summons four minions from his deck and grants them all taunt. While your deck has decent boardwipe potential, it doesn't take long to run out of options, and most of the time you're going to die long before you can summon Anetheron.

Mercenaries

• Icehowl, the penultimate boss of Winterspring, is one heck of a challenge. His attacks always hit the same targets, so he's easy to predict, but he has 50 attack, allowing him to basically 2-shot your entire team at worst (especially if they’re attackers). If that wasn't enough, he also turns anything he kills into an ice block, who's only abilities are to move to the left or right, which makes thing even more difficult. To top all of this off, he's encountered with two Chillwind Yetis, who soak up hits and can easily take out any casters you set up to kill him.

Other

• Gearmaster Mechazod, the boss of the "Unite Against Mechazod" Tavern Brawl. To start with, it's a cooperative brawl - both players must work together to take him down. If one player refuses to do so for any reason, whether intentionally or not, it's over. Once that hurdle is overcome, there's Mechazod himself, a minion with Taunt and 2/95 stats who switches what side of the board he's on at the end of each turn, as well as 5 special cards he uses: Overclock, which increases his damage by 2, Bomb Salvo, Prioritize, and Double Zap, which all deal his damage to varying targets, and Kill The Lorewalker, which destroys Lorewalker Cho (who is much more useful in this brawl than he is normally, making this a rather hefty loss). Mechazod has a habit of gradually using Overclock until his damage becomes too difficult to heal off, at which point it's a game of hoping he decides not to spam Double Zap, which can melt away at both player's health in record time. Finally, there's a quirk that the game doesn't tell you about until it happens: if the player has a full board right as Mechazod is about to switch sides, he flat out kills everything on their side of the field.