Current Record: 30-11 (73%)
Mecha'thun caught my eye immediately when Boomsday was announced. It seems to be a niche but powerful win condition so long as it's consistent. After release I toyed around with some of the early builds making their rounds and ended up enjoying the Warlock variant the most. I tinkered with super-draw versions (which included Loot Hoarder and Acolyte of Pain, standard control versions with Stonehill Defender and Doomsayer, and the even variant with Genn Greymane. In the end, none of them felt consistent enough to climb with.
The control matchup was never a problem. I found most of my issues were against popular meta decks like Deathrattle Hunter, Evenlock, Odd Rogue and Even Paladin. Often times I found myself falling too far behind in the early- and mid-game with a full hand. Fighting for board control was constantly a losing battle, and I'd eventually be too overwhelmed to draw my deck out.
After playing a dozen or so matches with each variant and discovering where the decks were falling short I decided to put my own together. I cut certain cards that felt dead most of the time or didn't have enough impact and focused on the fight for board control in the mid-game, and survivability in the end game. I also found my hero power dead often enough in the mid-game because of my hand size. Instead of relying on our hero power, we use cards to draw as fast as possible in the mid- and late-game because it thins out our hand and challenges board presence. There are currently 7 cards that will draw for us.
Vulgar Homunculus — This sees play in the Even variant of the deck, but not the others. I chose to include it because it's cheap, it's a strong early card for anti-aggro, it's a Demon (synergizing with DK) and it activates Lesser Amethyst Spellstone.
Voodoo Doll — Another card that sees play in certain variants. It's cost is cheap, and activates with Mortal Coil, Defile, Lord Godfrey, Hellfire or DK hero power. Excellent removal for Mountain Giants and other big threats.
Witchwood Piper — Thins out our deck regardless of when it's played which is important enough to include. It's also a Demon, and although the body isn't anything special, it's a bonus for DK. Being able to draw Kobold Librarian early for faster cycle, or Vulgar Homunculus to stall is also excellent.
Zilliax — The first card on this list that I haven't seen in another Mecha'thun deck. At first it might seem out of place or underwhelming, but it fits really well into the mid-game after playing Giggling Inventor, and it pairs well with Galvanizer. An excellent additional taunt, an excellent additional source of life and a removal that needs to be addressed.
Bloodreaver Gul'dan — This may seem counterintuitive to the draw archetype at first but I've found it works excellent towards the last few turns of the game. There aren't many Demons in this deck, just Vulgar Homunculus and Witchwood Piper, but we're not really using it for it's battlecry. Reviving two Vulgar Homunculus, gaining 5 armor, and having access to 3 damage lifesteal is an excellent long-term stall versus all archetypes, but specifically tempo and aggro.
More specifically, when the match is approaching it's late stages, survivability becomes more important than draw. You'll also find many times that with a few cards left in your deck you'll have enough cards in hand that draw for you, lowering the value of your hero power. And finally, using your vanilla hero power in the late stages of the game is often times too risky because of your low health. DK and it's hero power stabilizes all of that.
Doomsayer — This card is simply too weak in most matchups. At best it clears the board and stalls for one turn, but we're not necessarily focused on gaining board control, just preventing our opponent from gaining it. At the very least it soaks up 7 damage or is silenced/destroyed with no board impact. If it's sole purpose is to stall the early game, Vulgar Homunculus achieves the same thing but synergizes better with the deck.
Stonehill Defender — Another card that is simply too ineffective to be included. It's a weak body, it adds a card to our hand which we don't want because it limits our draw potential, and most of the usually powerful choices from it's Discover mechanic (Voidlord, Rin, the First Disciple, The Lich King) are too slow and counterproductive to our draw strategy (Rin and Lich King adding cards to our hand is a bad thing!).
Hooked Reaver — It's a very tempting include, but it's conditional taunt and lack of taunt when it's revived through the DK battlecry lowers it's value. We have enough mid-game taunt as is, and when we drop Bloodreaver Gul'dan we're not doing it for the bodies. NOTE: Currently testing a version with +2 Hooked Reaver, -2 Shroom Brewer. Brewer's definitely the easiest card to cut in this deck and I've been trying to find a way to get Reaver in.
Siphon Soul — It's expensive, it's slow, the life gain is negligible, and we have enough soft and hard removal already.
Mountain Giant — This card fits well into the deck in a lot of ways, but it just doesn't add much to the overall gameplan. Our early game is already defined with our low cost minions, our draws, our heals, our removal. Our mid-game is already defined through our taunts and removal. Our late-game is defined through our need for survival and drawing to our combo. It won't win us games singlehandedly and it doesn't offer enough to our eventual goal of drawing out.
The Soularium — Honestly, I don't have this card and I'm not sure I want to craft it so I don't know how well it would work in this deck. If I open up a copy I'll definitely toy around with it and report back.
Matchups, Mulligans, Etc.
Generally this plays like a Control Warlock deck with a hard win condition in the late-game. Draw often in the early game through your hero power and your draw cards. In the mid-game your hand is going to limit your draw, so focus on thinning it out by dropping your taunts, clearing the opponent's board and healing. Late game goes back to drawing and stalling. Be smart about what you choose to remove and what AOE cards you use, you don't want to run out of clearing options.
I can go into more specific matchups and mulligans if there's any demand for it, but this should be a good starting point. Leave me some feedback, ideas, suggestions, etc. if you want. Or don't. Just do you!