Hearthpwn

Voidwalker : A demon that many demonlocks don't run. This card greatly helps us when we're facing aggro paladins (especially token m/u) and aggro druid, which reduces buffed enemies for a good defile turn or just tanking damages. It then becomes a 3/5 taunt with Bloodreaver Gul'dan and Mal'Ganis and that rocks!

Dark Peddler : This 2 drop isn't a great body but lets us discover multiple cards that may be really helpful in some situations. Some examples : Power Overwhelming, Voidwalker, Mortal Coil, Possessed Villager for Defile shenanigans as well as
Elven Archer, Bloodsail Corsair and Hungry Crab for pesky paladins! Etc...

Gnomeferatu : Most people consider this card is a loss slot in the deck, a vanilla 2 drop that sucks. Well, it is. Until we face decks that have win condition that are very reliable on certain cards. Highlander priest have 1/3 or more of their cards that can make us win games easily when deleted. Every key cards in opponents' decks has a chance to be gone forever with this cutie. It's also a turn 2 play vs aggro that won't fuck us up (Dirty Rat pulls Tirion? Highmane?)

Dirty Rat : Hell yeah. We win versus exodia mage with one card. This card has made it into the deck for mage m/u and priest, secondarily for pirate warriors or keleseth rogues.. Your main targets are Raza the Chained,Spawn of Shadows, N'Zoth, the Corruptor, Kazakus, or battlecries that make them draw. We try to use it really fast against warrior when they still have 1-2 drops in hands because pulling something else is bad. When we don't have the option to, we should keep it for critical situations or to combo with aoe board clear.
Against rogue, use it fast if they kept their cards in mulligan or just be careful of 5/5 cards.

 Kabal Courier : It's an early game card that when drawn, helps us contain aggro boards early on if we can't find something else to do. Also, it lets us discover another Bloodreaver or Mage and Priest DKs which are damn good. It's not the best card for its slot but I still really like it and helped me alot during the grind. Its stats didn't have
any downside in my games.

Tar Creeper : Great 5 health taunt on turn 3. It's a pure anti aggro card that is almost useless in other m/us. Nothing else to say, it saves our face and lets us tap more to find further threats or board clears.

Mortal Coil : Didn't have a slot to put a 1 dmg card that can be a dead card when there are better damage dealing spell like Defile versus aggro.

Lord Jaraxxus : I really like Jaraxxus and its ways to make us win game that were going to end badly. It's surprisingly good when combined with Emperor Thaurissan, or if we manage to play it with 2 hp left against aggro decks. But I clearly don't need that card to win games. It also puts our total health to 15 even after we play Bloodreaver. That's really bad if we plan on going to very late games. Anyway, there are many reasons we want to play this card as much as we don't want to.

Bloodmage Thalnos, Tainted Zealot : those are cards that I sometimes considered to play (both of them) alongside Mortal Coil and Demonwrath, but that was too much conditions to take part into the deck. I mean having all required cards in hand, paying the mana cost for it was too slow versus aggro.

Brann Bronzebeard : A cutie to combine with Kabal Courier, Dirty Rat, Gnomeferatu,Kazakus, Twilight Drake etc. But for what? To overbeat control decks. That, we don't need. Bloodreaver wins games on its own! It is way too slow against aggro decks aswell!

Antique Healbot : Paying 5 for a 3/3 body that does nothing but letting you live one last turn more is not our business. This card is not consistent in every m/us, and doesn't help getting the control of the board back. I never needed to heal 8 on my grind to legend.

Dirty Rat : Sunfury Protector, Acidic Swamp Ooze, Sludge Belcher

Gnomeferatu : Sunfury Protector, Acidic Swamp Ooze, Antique Healbot, Brann Bronzebeard

Emperor Thaurissan : Skulking Geist,Antique Healbot, Brann Bronzebeard, Sludge Belcher, Sunwalker

The Lich King : Lord Jaraxxus, Primordial Drake

Those are some ideas. Feel free to make your own deck if you want to replace something else!

MAGE : Most of the time it's exodia mage. You want to keep Dirty Rat, Krul the Unshackled, Mountain Giant...
Quite akward because the whole match up is about whether you kill him before he draws combo or you ruin it with Dirty Rat.

PALADIN : Tokens and Murloc paladins are two very different things. Once you get rid of important murlocs, you basically win by the value of your cards. Against tokens, you need to be careful when using ressources to kill their tokens because they have a lot of mass buffing cards like Sunkeeper Tarim, Lightfused Stegodon, Quartermaster. You want to keep Mistress of Mixtures OR Voidwalker, Dirty Rat, Defile, Dark Peddler, Darkbomb, Tar Creeper, Despicable Dreadlord against token.
Against murloc, prioritize Mistress of Mixtures,
Doomsayer, Reno Jackson, Defile, Darkbomb over the other cards listed up.

ROGUE : You will most likely face Prince rogue. Mulligan for Mistress of Mixtures OR Voidwalker, Doomsayer, Gnomeferatu, Darkbomb, Kazakus. You just have to clear their minions, especially before turn 7 to prevent Bonemare buff.

WARLOCK : Against Zoo, keep Mistress of Mixtures OR Voidwalker, Doomsayer, Darkbomb, Tar Creeper, Hellfire, Reno Jackson. Basic board fight.
For mirror m/u, keep Mountain Giant, Twilight Drake, Voidcaller if you have a good demon already, Kazakus, Darkbomb. Usually, you have more chances to face demonlocks at high ranks than zoo. Against these ones, you need to aggressively play you threats and try to have a potion that resurrects you demons, summons one, or polymorph all enemies for his Krul turn. That's the global idea. Make him use his removals before Bloodreaver if the game goes long enough. Or try killing him fast!

PRIEST : Well there is a shit ton of priest styles of decks around in wild. You can meet big priests -easiest m/u-, dragons, divine spirit/inner fire, and 50 variations of Highlanders. The Highlander that run OTK cards like Spawn of Shadows or Recruiter or a bunch of 1 cost spells are difficult to beat. But there are also ones that don't, thank god. When against Highlander, mulligan for Mountain Giant, Twilight Drake, Dirty Rat, Krul the Unshackled. Try to pressure them fast, pull Raza the Chained with rat and win. Do not play Bloodreaver Gul'dan into Shadowreaper Anduin!!!
Against Big Priests, Kazakus is the hero here with his polymorphing potion. You need to make him summon as much minions as you can to make them sheeps so he can't revive them. The Obsidian Statue is not a threat, which mean you should not use you removal spells on those. This fight is about value.
Don't keep Dirty Rat.

SHAMAN : I faced a few aggro shaman rushing my face so fast I just couldn't survive long enough. Maybe the evolve m/u is easier. Look for Mistress of Mixtures, Doomsayer, Darkbomb, Hellfire,
Blastcrystal Potion, Defile. Always keep Reno Jackson!

WARRIOR : Pirates it is. Keep Mistress of Mixtures, Doomsayer, Voidwalker, Dirty Rat, Kazakus, Darkbomb. Always keep Reno Jackson!

DRUID : Against aggro, keep Mistress of Mixtures OR Voidwalker, Doomsayer, Dirty Rat, Defile, Reno Jackson, Dark Peddler. Easy game if you can just clear the board. Keep aoe like Defile if you didn't use it, Hellfire or Kazakus 1/5 mana potion to clear Living Mana. You can face jade but I never did. Put the maximum pressure on the board, your capacity to take control of the game is superior. Krul or Bloodreaver is usually the win for you.

HUNTER : Look for Mistress of Mixtures OR Voidwalker, Doomsayer Dark Peddler, Defile, Reno Jackson, Defile, Kazakus. Watch out for playing Doomsayer into Kindly Grandmother.