Hearthpwn

This is the main win condition of this archetype, as it has always been. This pure playstyle can be only adopted against classic control and slower midrange decks. I will discuss about specific matchup strategies below.

By definition, the aim of the deck is to fill the opponent's hand and burn the overdrawn cards, killing him by fatigue later in the game, exploiting the deck size advantage built through Gang Up and Deathlord.

The key card to achieve this is Coldlight Oracle. You are looking for this card in the mulligan, and you are not going to play it until you can ensure to use Shadowstep or, at least, Gang Up on it. Using the oracle hoping to draw this exact spells is a risky play, but in some rare cases, acceptable. If, for some reason, you lose both oracles, your main win condition is gone.

You need to stall the game clearing the board with your taunts and spells. As we will see, Vanish and Sap are particularly important. Use damage dealing spells for this purpose and not in your opponent's face, except as a finisher.

At least one, possibly both Gang Ups must be played on Coldlight Oracle. There are very few exception to this (in some particular matchups you may want to use it on N'Zoth, the Corruptor or Antique Healbot), but however, never think of using it on another random opponent's minion, for how much strong it may seem.

Brann Bronzebeard can be kept as a finisher to draw up to 12 cards, in combo with the oracle. If necessary, it can be used earlier in the game with no particular drawbacks.

Deathlord should be played very carefylly. If you are facing a slow deck, you want to play it only if you are ready to immediately remove/bounce the powercreep it could pull out with Sap or Vanish.

Be careful to not mill your own cards if it not strictly necessary. You could end up burning key cards and screw up the game. For that reason, be sure to end the turn with less than 10 cards. Be particularly careful in the turns when you are using Vanish, since the bounced minions can fill your hand.

Vanish and Sap are the main removals of this type of deck. To use them efficiently you need to be aware of how do they work.

In a nutshell, when you bounch back a minion in its owner's hand, if the hand is full (10 cards already), the minion will die and activate its deathrattle.

When using Vanish, this mechanics becomes hard to manage: the minions that were played first, will be bounced back, the rest that do not fit in the hand, will be killed. You can manipulate the outcome with Coldlight Oracle: if you want to kill more minions, play the oracle first, and then use Vanish. This mechanics applies also to your hand, so be careful not to kill your own key minions.

Let's understand which kind of your opponent's minion you want to bounce, and which you absolutely need to kill. Obviously, you should try to bounce in hand high cost minions with dangerous deathrattles (Tirion Fordring, Shredders, White Eyes, Savannah Highmane...). Instead, you absolutely need to kill minions with charge (including Ragnaros the Firelord), with dangerous battlecries (Loatheb, Dr. Boom, Drakonid Operative, jade-spawning minions, Old Gods), and minions that can be played next turn for a ridiculously low amount of mana (Thing from Below, Giants...).

I want to warn you here that knowing when to use Vanish makes the difference between a win and a crushing loss, and between a skilled player and a novice. The most common misplay I observe from people playing this deck is using Vanish too early on an absolute worthless board, that can be easily be dealt with just a taunt and maybe some other inferior spell. Always remember that, except some cases, this card is the most powerful tool you have to control the game, but you only have two of them, and it's not guaranteed that you will draw both as soon as needed. So play it when your opponent's board gets to be really threatening. 

When possible, play Doomsayer on the empty board following Vanish. It will earn you another turn for your strategy, or force hard removals on such a worthless minion.

Main win condition of this deck is using fatigue to kill your opponent. Virtually, you have the chance to draw a total of 12 cards in only one turn if you have the right combo in yout hand (Brann Bronzebeard, 2xColdlight Oracle, Shadowstep). Usually bringing the opponent to fatigue for 7 is enough to kill him. So sometimes you could have a lethal when your opponent has still 4 or 5 cards left in his deck. It's really easy to miss lethal with this deck, let's try to avoid it by learning to count.

The general formula to know how much damage a player will take by fatigue is x*(x+1)/2. However, you could just remember the series 1,3,6,10,15,21,28. This won't be enough while facing control warrior with plenty of armor. When you count this, remember that your opponent will take also one more fatigue damage at the start of his next turn.

Of course the same thing applies to you. You need to make sure you won't kill yourself. For example: you have the combo in your hand to kill your opponent. 12 card draws will also kill you, but 10 will let you survive still killing your opponent. Then you play the combo in another order (Coldlight first, then Brann and the other two Coldlights) to draw only 10 cards.

Let's see how we face the mulligan for this deck. 

As we know, mulligan is a gamble, because we cannot be 100% sure of what specific archetype our opponent is playing, so I won't bother describing mulligan for each matchup. I will write some general tips that you need to adapt to the meta you are facing.

In general, when we face a particular class, we assume it is the most common archetype played of that class currently. If more archetypes are played, we assume that it is the worst matchup for us, that is aggro.

The card that we always keep is Doomsayer. It's perfect for early game in nearly every case.

Coldlight Oracle is the key card that you should usually keep. In the case you expect an hyper aggressive deck (Pirate Warrior, Token Druid), change it immediately. Even if we end up facing a Control Warrior, we will have time to draw the murloc later.

As I said, when we are assuming an aggro mathchup, your first choices in the mulligan are Doomsayer, Deathlord (and Acidic Swamp Ooze and Golakka Crawler for Pirate Warrior). Early game removal like Backstab and Eviscerate are ok. Also Antique Healbot and Sludge Belcher are ok, but they cost too much and together with them you absolutely need something cheaper to be played before.

You basically never keep Gang Up, but you can keep Shadowstep if you also have a Coldlight Oracle, against slower decks.

Vanish is a card that generally you should change, of course because of its cost. Anyway, particularly against midrange decks as Paladin, Shaman and some Priests, you could decide to keep it as plan B if you horribly lose board control in early game, if the rest of starting hand is not complete lame.

Aggro decks will try to vomit their low cost minions and spell to your face. Your playstyle needs to be changed accordingly.

The perfect combination to win is turn 2 Doomsayer - turn 3 Deathlord. Usually, coining out the hero power is recommended to face some turn 1 threats that risk to snowball, like Patches the Pirate or Living Roots.

Coldlight Oracle will be a dangerous card that draws your opponent burst damage. You don't want to use it unless in desperate situations where you have nothing better to play and you want to look for taunts or healbot. Also Vanish will have no use in this matchup, except rare circumstances.

Shadowstep needs to be saved for the Antique Healbot. It's also good to replay Deathlord when it's at low health, and (for pirate warrior matchup) oozes and Golakka Crawler.

Gang Up can be used when you have some "free" mana to spend, usually on the healbot, above all if you are facing a hunter or an opponent with hunter hero power. 

If you get to the point to play N'Zoth, the Corruptor with a decent amount of health, you have won.

The match is all about board control. The key card for this purpose is Vanish. Other fundamental cards, for the same reason, are Doomsayer, Betrayal, and the other single target removals. Deathlord is a perfect play if you have Vanish in your hand, otherwise you should be careful.

According to what card you draw first during the game, and how fast your opponent is, your strategy can vary from a pure mill playstyle, to a N'Zoth focused one.

These matches require the maximum attention. The strategy of your opponent is about drawing the pieces of his combo as soon as possible. We don't want to help him in his aim.

The best approach is usually to hide the nature of our deck until your opponent fills his own hand. At that point, we should try to draw him as many cards as possible in one turn. If we are lucky, we will burn immediately some pieces of his combo. At that point, just go on with milling: more cards we burn, more probabilities we have to burn his pieces.

In this matchups Antique Healbot has a critical role. If you are facing a deck that requires a preparatory turn with Alexstrasza (freeze mage), you need to save it to use it immediately your next turn. If you are facing an OTK deck (usually Malygos decks), you should be able to burn at least some part of it: your opponent should have then a "crippled" combo that needs to kill you in more than one turn, and the healbot can ruin this plan.

If the combo deck you are facing is not based on spells that can diretly go to your face (Anyfin Paladin, Aviana-Kun Druid), your strategy needs to revolve around N'Zoth, the Corruptor, as your opponent's combo is nullified in front of our taunt wall.

The first thing I could suggest is to burn the card. Usually the druid will use the first one to summon a golem. If he does, you could try to burn the other one. 99% of the times this won't work out. Let's move on.

The main cards in this matchups that we are searching for are the taunts, Vanish, and N'Zoth, the Corruptor. Coldlight Oracle is needed only to draw them, as you cannot possibly mill the opponent.

We need to try to mulligan as many as taunts as possible. Remove jade spawning minions as soon as possible (you don't want to use vanish on them later). Also remove immediately Fandral Staghelm and, above all, Gadgetzan Auctioneer. 

Stall the game with taunts and spells (Betrayal will be a very useful card) until you get N'Zoth. Save a Shadowstep and a Gang Up to use on it. Do not use both Vanish before this point, as you absolutely need one in late game. 

If you get to the point to play N'Zoth and you didn't waste both Vanish, you have very good chances now. At this point the druid will realize what is going on and will use the idols to spawn minions and remove your taunts. Keep replaying N'Zoth when he manages to clear your board. Meanwhile, remove his lower level jade golems. 

When the board is filled with 10 mana fatties, use Vanish. Your opponents hand will be filled with 10 mana minions that can't be played together with another Jade idol. Next turn you will play N'Zoth and refill your board, that your opponent won't be able to clear anymore. At this point you should go face. If, for some reason, he can rebuild a decent board before you win, always be careful that he won't be able to kill you next turn. If he could, crash some of your deathlords and replay N'Zoth, while you go face exploiting the wall of taunts.

Let's see what cards you can decide to replace in order to be prepared for meta shifts, or just because you don't own them. The cards listed in the deck that I won't include below are staple cards that should not be removed. In this section I will also discuss about some "false friends" that I strongly discourage choosing.

• Preparation has an insane tempo value, but in a fast meta it's the card you never want to see as your topdeck.
• Betrayal is very good against midrange decks. Still useful against aggro. Fundamental against any jade deck. While jade will still be a thing, absolutely run 2 copies of it.
• Backstab is commonly used, but in the current meta I have found it a little underwhelming to face aggro decks.
• Shadow Strike is preferred over 1 Eviscerate because it can directly deal with widely used minions like Emperor Thaurissan, Thing from Below, Jade Chieftain, Aviana, Auchenai Soulpriest and so on.
• Deadly Poison and Blade Flurry were a commonly used combo in this deck. Not really viable anymore because of the nerf.
• Conceal's only purpose in this deck is to hide Doomsayer. It's a great move to clear the board, but you will take tons of face damage. It cannot be possibly used to hide Coldlight Oracle, because every aoe spell can easily kill a 2 health minion.
• Sabotage is not a terrible card, above all in this weapon-filled meta. Still, the high cost to kill a random minion is a big drawback. 
• Beneath the Grounds is terrible against any deck of which you should be able to burn some card. In fact, you could burn Ambush!, losing its effect and saving other opponent's cards. Against the other decks (mostly aggro), it's just a huge waste of tempo. 
• Sprint is a dead card in most matchups, as it destroys your own win condition. In aggro matchups it could theorically be useful if you get late game, but its cost is such a tempo waste that you will probably lose either way. 
• The Caverns Below is a card I do not recommend. You don't expect having any board to take advantage of it until you play N'Zoth, and even then, the effect won't change the outcome of the match. It only steals a slot for mulligan.
• Shadowblade could be a good card for the mulligan against aggro, 3 damage can remove many of early game threats. It's basically the same play as Hero power-Deadly Poison, except you save yourself some face damage. Plus, it may have a "Violet Illusionist"-like usage to avoid fatigue damage, good for mirror matches.

• Brann Bronzebeard gives a huge boost. It can work as finisher with the coldlights, or heal booster with the healbot. The only reason you should remove it is that you miss the LoE expansion.
• The Curator gives the deck more consistency. Drawing the oracle early enough is your win condition in most matchups. With this card you basically have 3 oracles instead of 2. It's a taunt, so not completely useless in aggro matchups. Beware that, if you run a beast (crabs) and/or dragons in the deck, they will be drawn before the murlocs, so be careful not to burn the oracle if your hand is too full. For a replacement, see Azure Drake.
• Skulking Geist is the ultimate tech card against Jade Druid, Secret Paladin, Evolve Shaman and a lot of combo decks (Malygos, Combo Priest...). Its only existence can be a deterrent to play any deck that relies on 1-mana spell cards, so there could be some moments in the meta when you can decide to remove it, because none will be playing the card that it is supposed to burn.
• Acidic Swamp Ooze is the tech card to counter pirate and aggro decks. If, as we all hope, the meta will drastically change, you could choose to remove one or both.
• Gluttonous Ooze is the new, better version of the previous one. I use one of both because, sometimes, the 1 mana difference can be decisive.
• Golakka Crawler is a must have in the pirate meta. It increases your winrate against pirate warrior by A LOT. Just a random vanilla 2 drop in other matches.
• Hungry Crab for a guaranteed ownage of paladins and co. in a "murlocs everywhere" meta.
• Bloodmage Thalnos is a classic mill rogue card, but it has a very bad synergy with N'Zoth, the Corruptor. If you decide to run it, be careful to NEVER use it in some matchups (jade druid, mirror matches, other fatigue decks).
• SI:7 Agent is, in general, a very strong card. Unfortunately the 3 mana slot is overwhelmed in this deck, and the combo effect is not always easy to pull out in early game. 
• Shadowcaster has a very interesting concept for this deck. Its combo with Brann Bronzebeard and N'Zoth, the Corruptor (and any other battlecry minion useful for the matchup) would be awesome, but in this moment it is just too slow.
• Youthful Brewmaster is basically a Shadowstep with a body. The 2 drop body can be useful, but the missing mana reduction is a big drawback.
• Refreshment Vendor is recommended over Earthen Ring Farseer for its bigger body and better heal. Also the 4 mana slot of this deck is empty, so it can be a good 4th turn play.
• Prince Valanar can cover the same role of 4 mana filler, with the advantage of the taunt. Nevertheless, differently from Refreshment Vendor, if it gets removed by a spell you lose the heal.
• Dark Iron Skulker is a very good card against zoo. Unfortunately it's pretty slow and completely useless in all other matchups. It will be a nice addition if ever zoo will see a rebirth.
• Azure Drake is another classic mill rogue card. Its properties can be helpful, but digging it out with The Curator can have the important drawbacks to reduce your deck size and fill too much your hand. So avoid running them together.
• Xaril, Poisoned Mind's Toxins are pretty interesting, but I don't feel there are cards worth to be removed to make space for this. As a deathrattle minion, interferes with the Jade Druid strategy (see above).
• Violet Illusionist can be used in the finishing combo to not take the oracle fatigue damage, but it's useful only in mirror matches, as there is not another deck that can possibly compete in fatigue with you own (except Jade Druid of course).
• Unstable Ghoul is a good card against aggro, and I definitely recommend it. But If you revive it multiple times with N'Zoth, it's going to be a weak spot in our taunt wall.
• Mistress of Mixtures is a very good 1 drop in aggro matchup. As a deathrattle minion, interferes with the Jade Druid strategy (see above).
• Dancing Swords could be good against control and fatigue decks, but those are already our best matchups. Drawing a card for other type of decks is not advised. As a deathrattle minion, interferes with the Jade Druid strategy (see above).
• Loatheb is a life saver against spell based aggro decks (like tempo mage) or combo decks (Malygos decks, freeze mage). Also, in general, it's the perfect card that you want to play the turn before using Vanish.
• Dirty Rat can be considered if the meta switch towards those combo decks that are hard countered by it. Against aggro should be ok, particularly if you use it when you can deal with the pulled minion (for example with Betrayal). However it's not synergistic with the mill strategy, since our aim is to fill opponent's hand, not to slim it down.
• Emperor Thaurissan could theoretically enable some crazy combos, but it's useless in aggro matchups and, in general, its high cost-body ratio puts in danger your board control.
• Big Game Hunter is another card that lost his role after its nerf. It is usually useful in those matchups where you are already favored, and it is terrible in the worst matchups (aggro).
• Vilespine Slayer is the new card that can replace Big Game Hunter, removing the 7-attack limitation. It's definitely a good card, in the right meta.
• King Mukla can enable some burning card combo by filling opponent's hand with Bananas. Just as other cards in this list, it's very good in already favored matchups (control), and bad against worst matchups (aggro).
• Nat, the Darkfisher is more like a meme card. It shares the issue of Dancing Swords: you don't want to draw cards to aggro decks.
• Reno Jackson has no point in this deck. It's useless against aggro, and can be active only in late game against control decks, at some point of the match when you should have already won. 
• Barnes. Meh. In combo with Shadowstep, 80% of minions it can summon are great targets to bounce to your hand.