Hearthpwn

Leviathan is really bulky, reminiscent of Loatheb. And like Loatheb, it has a way of taking it down a few notches. Leviathan also uses a deck centered around Mechs and Mage fire spells. Let's start with its hero power:

Leviathan will be dealing constant damage to your tokens every turn, so Aggro isn't gonna work. Additionally, it'll start the match with a turret and resummons it again if you destroy it. This is both a blessing and a curse, as you'll see here:

It'll add on to the damage you're taking every turn, but it's also your key to breaking through the Leviathan's massive health pool. When destroyed, you get a reprieve from the assault and get the chance to hit the boss where it hurts as you overload its circuits for a turn. To help you out, you also get a card at the start of the fight:

This will let you pick any one of these three cards to help you out. Pick the one that fits your strategy the best - one suits focusing the turret, one benefits spell casters, and the last will let you tank it out.

Each one's built to last awhile, but eventually you'll have to make do without them unless you keep them healed. This isn't made any easier by the medley of custom cards Leviathan uses:

This'll keep you from stat-boosting your starter vehicles too much, lest they get blown right off the field. Everything else will make it a bit rougher to keep the turrets down so you can focus on the boss.

Intro:  Brann: Bronzebeard: Glad ta see ya again, explorer! We'll need your help if we're ever gonna crack the mysteries of this place. Be wary, I'm sure they still have some nasty defenses laying arou- WHAT IN BLAZES IS THAT?! EVASIVE ACTION!

Fight Start: Target locked. 

Emote: Not programmed with communication protocols.  

Turret Defeated (1st time): ZGRWXWRRR. Circuits overloaded. Activating support systems. 

Heroic Card activated: Orbital countermeasures enabled.

Defeated: Error. ERROR. ERRORERRORERRERRERR (Explodes) 

Brann: That was only the first boss? Boy, are we in for a ride.

On heroic, Leviathan's health pool grows even bigger, meaning you'll have to hit harder or last longer to take him down. His turrets and spells stay the same, thankfully, but its hero power...

...Will be hitting even harder, as well as slowly chipping away at your hero. And that's the least of your worries. On heroic, Leviathan gains FOUR LEGENDARIES that can potentially come into play from turn 5 onwards. They can't attack, but if not dealt with quick their effects will leave you helpless.

Whether they're destroying, halting, damaging, or outnumbering your forces, you're in trouble if they're not dealt with quickly.

For surviving the onslaught, you get to rip off one of the machine's turrets and take it home with you - without the increased damage, thankfully.

'RA-TAT-TAT-TAT-TAT'

As for the class challenge, you'll be given an all mech Mage deck to face off against the Leviathan's. You'll be given a new card as well to buff up your forces, and when you win, you get to add it to your Mage deck:

'This is also what musicians call cleaning their instruments.'

Ignis is also heavy on the Mage flame cards, though he also has a knack for Warrior abilities and weapons. His hero power also puts a spin on things:

If he has the mana, he'll dunk your hero in his pot, covering your portrait in a red-tinged shadowform effect. This disables your hero power, so make sure you build a deck that doesn't rely on it. Ignis also has a few spells of his own, plus a minion that's strengthened as he burns your tokens to ash:

You'll have to place your minions strategically and prepare to lose some of your bulkier ones occasionally. Just make sure to deal with those constructs before they heat up too much.

Intro: Brann: Alright, if we want to make it any further in we're going to have to get past the legions of iron infantry. Or, ya know, we could just cut them off at the source. Whatever suits your fancy.

Fight start: You dare step foot in my workshop? You will burn! 

Emote: Begone! 

1st Hero Power use: I WILL BURN AWAY YOUR IMPURITIES! 

Defeat: Who will... take care... of my creations?

Brann: Ach... is it hot in here, or is it just me? Let's get a move on before I keel over. I mean - you keel over.

Ignis gets a bit bulkier on heroic, and his deck/AI are a lot better, but for the most part his spells stay the same. One major difference, though, is you can't use your hero power at all. Don't build around it.

Once Ignis is all burnt out, you get to steal one of his constructs to take home with you. Sweet.

'Little known fact: It's also programmed to make a mean Magma Shake.'

Razorscale is a two-phase fight and she starts the fight Immune and with 5 armor. Her deck is stuffed with dragon cards, as well as some dwarven helpers and fire spells. You can't touch her with anything you have - you'll have to rely on her minions to do the work for you. While you work on destroying her armor, you'll have to contend with her hero power:

When used, she'll bash one of your tokens, flinging it into your hero's face and hurting you both. She'll continue knocking you around until you get rid of the armor and put her into phase 2, which you can do by killing her dwarves:

You'll need to defeat at least one watcher, and the rest can be any mixture of dwarves that would deal 5 damage to her. Once her armor's gone, she loses immune and enters phase 2, replacing her hero power with this:

This works as an enrage, and you'll need to defeat her before she becomes too strong to defeat. Additionally, she uses a couple custom spells to harass you:

Yep, she won't think twice about roasting her own minions. She'll tend to save it for emergencies, though.

Intro: Brann: Alright, we've got one last thing in our way - and it's a big one. Razorscale's flying up high, and we can't get any further until she's out of our way. Bring her down.

Fight start: Razorscale: (Dragon screeching)

 Brann: Whoa, she's way up! There's no way we'll bring her down alone. Maybe she'll play something that can help us...?

Emote: (Roar) 

Phase 2: Brann: Whoa whoa, look out, she's coming down! Oof, that looked painful! Alright, now's your chance... take her out! 

Defeat: Brann: Alright, ya did it! The courtyard's clear. According to the scouts, there's just one last thing between us and the inner city - come find me when you're ready. 

Razorscale's not much bulkier on heroic, but the real problem comes from her now having 8 armor - that means you'll have to defeat every one of her dwarves to bring her down. To keep it from being too RNG heavy, she'll always have them near the front of her deck, but you'll still have to deal with an overall stronger card selection (and a thankfully exact same hero power). Phase 2, however, toughens up her second power as well as adding a new spell:

This gives you less time to finish her off, and will occasionally make her minions too strong to get destroyed by her Devouring Flame.

If you tough it out, you get the ability to add some Dark Rune Dwarves of your own to your arsenal. Again, they're now rendered harmless to the user and retain their other effects. The Watcher's battlecry will now activate as soon as it's summoned, though.

'The leading cause of injuries among dwarven ranks is accidentally drilling into people's bathrooms.'

Additionally, for completing the entire first wing, you get to add Razorscale herself to your deck as a strong-statted legendary. She also brings an entire roster of dwarves with her to give you a lot of board control, albeit some rather fragile control.

'Razorscale and the Three Dwarves. Yeah... has a nice ring to it.'

Lastly, the final class challenge for this wing pits an overload-focused shaman against Razorscale once more. Defeating her again grants you a new card that gives you some early-game randomized burst:

 'You spin me right round, baby, right round.'

Merely a Flesh Wound: Deal 10 damage with a single attack.

Power Overwhelming!: Deal 20 damage with a single attack.

Nerf Incoming: Deal 30 damage with a single attack.

How the Mighty Have Fallen: Have one of your legendaries polymorphed/hexed.

Face is the Place: Win a match without killing any enemy minions.

NEVER LUCKY: Get Milhouse Manastorm out of Sneed's Old Shredder

Insult to Injury: Use Alexstraza to heal an enemy hero below 15 health, and then kill them on the same turn.

The Underdog: Have one of your 1-cost minions become the winner of a Warrior's Brawl.

The Ancient Win: Defeat an enemy hero by hitting them with The Ancient One.

Devoted Cultist: Boost your C'thun up to at least 30/30.

Ready for Adventure: Have Reno, Brann, Finley, and Elise on the board at the same time.

Backfiring: Have Tinkmaster Overspark turn any minion below 5/5 into a Devilsaur.

Why Are you Hitting Yourself?: Defeat an enemy hero with a card you stole from them. (Thoughtsteal, mindgames, mind control, mirror entity, sylvanas, huckster, burgle, etc.)

Downgrade: Have you Piloted Shredder or Sky Golem summon another Mech.

Polarity Shift: Hit Thaddius with Crazed Alchemist's Battlecry.

SO HOT.: Defeat your opponent as Ragnaros the Firelord.

YOU FACE JARAXXUS: Defeat your opponent as Jaraxxus.

Tough Luck, Pal: Completely clear your opponent's side of the board with one spell when they own at least six minions.