I piloted this list from rank 5 zero stars to legend over the past couple of days. In that time, I lost a total of 4 games; by my math that's 29-4. Not sure if I'm going to keep going and try to get to high legend but will update if I do.
I'm not necessarily *sure* this list breaks the meta, but I kind of suspect it does in the sense that it will probably push control warrior out completely since it has a similar matchup profile but cannot lose the "mirror".
The win condition for the deck is for the following to all be true:
1) You have previously played Mecha'thun and he has died. We'll tell your story, Mecha!
2) You have no cards in your deck.
4) You have no other minions.
5) You have not played any deathrattle minions that create minions when they die.
Basically, you play like control warrior: you draw through your deck stacking up armor and trading with their board, and then at some point (almost always after Boom is down) you play Mecha'thun and he eats a guy, they kill him, and then you exhaust your deck and win with Undatakah.
Some specific card commentary:
Inner Rage/Whirlwind: Playing three copies of this part of your win condition means that you don't need to hold the first two. Neither is a great card on its own merits, but they do do useful things, especially against one-health minions; they also cycle off of Acolyte of Pain and enable Battle Rage.
Devastate: We play two copies of this because we want the first one we draw to not be a dead card all game. Feel free to kill something with it!
Shield Slam: Your best point removal. Note that you can juice this up in a hurry with Weapons Project or Shield Block if something big needs to die now. Try to hold this for something big if you can but it's fine to play it early if the alternative is going to cost you too much life.
Town Crier: Often is correct to hold this a bit in the midgame so that you can whirlwind/warpath it and get a card off of Battle Rage, but you slam it on t1 if you have it. This sets up your early game of Militia Captains very nicely, finds you Zilliax for one of your Killer Kombos, and shrinks your deck.
Battle Rage: It's possible these should be Slams, but Militia Captainenables them quite nicely and so do Crier/Aco with your whirlwind and ping effects. Potent card draw. You actively want to take a little health damage early to turn these on, so long as you can do so without letting the board run away from you. After you Scrappy combo these are worse, so try to play them before you do that, all other things equal.
Sweeping Strikes: In addition to being an important part of one of your Killer Kombos this is generally acceptable removal. Goes fine on a Militia Captain or on a post-Boom Scraphound but mostly it's there for the big Zilliax play.
Vicious Scraphound: You very rarely play this on t2-- Scrappy-Doo is for comboing with Zilliax and/or Sweeping Strikes to make a sweeper plus super-Reno. Has rush post-Boom and can often be productively Inner Raged or Sweeping Struck at that point without Zilliax involved. You play two to hit the Zilliax combo as quickly and consistently as possible and also so that you can dump one to manage handsize or to contest the board early if you have to.
Warpath: Best sweeper around. Try to hang on to these since they also hard enable draw off Acolytes and since they're a great answer to anything that isn't enormous.
Weapons Project: Armor, board control against aggressive decks, and weapons hate, plus the very potent combo with Harrison Jones. I don't usually play these t2 either except against very aggressive decks. Try to hang on to something that kills a weapon against rogue until their picks are gone. Again, we play two copies for combo consistency and because it's a strong, inexpensive card even outside the combo.
Acolyte of Pain: Be careful not to put yourself in a situation where your opponent can make you overdraw unless you already have everything you need. Not really an early play but if you got the mana and the time go nuts. You would ideally like to get at least two cards off of each of these. Obvious synergy with Whirlwind/Warpathand Inner Rage.
Shield Block: Often correct to just HP instead of playing Shield Block, especially if you haven't taken any health damage yet.
Militia Commander: These contest the board for you in the midgame and leave a body your opponent will then spend tempo handling.
Harrison Jones: Mostly in there to draw three cards off of Weapons Projectbut also eats wrenches and picks sometimes.
Zilliax: Amazing card generally but in this deck you really want to try to get more than normal value. Fuse it to a Vicious Scraphound and your seven mana Scrappy-Doo gets you 5 armor, 5 life, and a 5/4 taunt that gains you another 5 armor/life if they have to interact with it in combat. Fuse it to a Scraphound and play Sweeping Strikes on it for nine mana and you probably just killed three minions, gained 15 armor, gained 15 life, and they still have the actual body to deal with. Do not play this without Scraphound unless you absolutely have to do so.
Dr Boom, Mad Genius: Enables rush on your mechs and turbocharges your hero power, plus gives you a bunch of sweet sweet armor. Skip the discover-a-robot HP unless you are sure you have plenty of mana to dump it-- remember, we need to play every card we see to win the game. If you do discover a robot, *avoid token generating deathrattles* or your win condition gets a lot worse.
Da Undatakah: Third-to-last card you play.
Mecha'thun: Play this whenever convenient but ideally after Boom when it has rush and works as a reasonable removal dude.
You are favored against all builds of rogue I've seen, although Myra's Unstable Element/Nomi is bad news for you and Myra's/Nomi/Shadowstep will probably beat you.
You basically cannot lose against control warrior or dragon warrior. Your game plan just beats theirs and their only hope is to overwhelm you with bodies off of Dragon Roar/Omega Protocol, which they have plenty of but for which they're paying full price.
You are somewhat favored against bomb warrior but it's a tricky matchup. Try to maximize the gain off of your Zilliax turn since you are going to eat every bomb they put in your deck-- hold your armor gain spells until later since you want to take some bombs on your health before you Zilliax that life back. Do not allow Elekk or Wrench to live, ever.
I managed to play against nearly no druids with this build of the deck on the way to legend. I suspect from playing against token druid with previous versions of the deck it is a pretty even matchup-- we have a *lot* of sweepers and can fight their boards pretty effectively with Sweeping Strikes and Captain and Weapons Project, but they have a lot of dudes and if we're not careful with our answers they can overwhelm us before we clock them.
Conjurer Mage seems like it should be a bad matchup but I beat like three of them and didn't lose to any, so *shrug*. My guess is that it actually is bad because what we are not good at is killing lots of board made up of multiple huge minions and that's kind of what they do, but they aren't going to run you over before you get set up or anything. Try to save Shield Slams for later-- their Twilight Drake on t4 isn't going to threaten kill pressure but the turn they play 4 Mountain Giants definitely is. Polymorph would be very bad news but nobody plays it. Research Project would be moderately bad news but nobody plays it either.
Hunter seems like it should be bad but again I beat a bunch of them and didn't lose to any. The aggressive build of mech hunter with lots of goblin bomb shenanigans is definitely better for us than the build that just wants to play Nine Lives on Mechanical Whelp a bunch. Sweeping Strikes is insane when the guy you're attacking is 0/2. I'm pretty sure that beast hunter can beat us by setting up a bunch of nasty rhino turns late, but nobody found that line against me, and it's possible that it's too slow anyway. I did not play against Malygos Hunter but intuitively it seems like it should be pretty favorable-- you gain a ton of armor and their Big Zul'jin Turn can be answered by Warpath.
You are favored against zoolock but they can beat you with a fast draw if you stumble and Rafaam turns it into a grind. Try to hold Shield Slam for killing something big if possible-- a giant, maybe a Carpet, Rafaam or something downstream of Rafaam.
Control shaman is literally a coin flip. If you have the coin, you can and should Shield Slam your own Mecha'thun. If you don't have the coin, then Mecha is very likely getting Hexed and then you lose. I guess there's probably part of the distribution where you have the coin but they run you over with stupid deathrattle tricks on large elemental men, and there's probably part of the distribution where Mecha somehow doesn't get Hexed (try to bait it out I guess?) or you pull another Mecha or a Galvanizer or something off of Boom, but most of the time the coinflip is going to decide this.
Murloc shaman is a pretty good matchup. Don't let the 2/3 dude hang around or they can overwhelm you in the midgame, but if you can kill the first two or three of those on sight they're going to run out of gas.
Secret paladin is a good matchup.
I suspect that mechapaladin isn't-- see previous thoughts about Very Large Guys and why we don't want to see too many of them in any given game--but I didn't see any so can't confirm.
Resurrection Priest is probably really bad for us but it also doesn't exist so *shrug*
Some general deck notes:
You can play two copies of Sweeping Strikes on the same guy and they will both trigger, hitting the side guys twice. This will proc lifesteal/armorsteal twice even if the first hit is enough to kill the guy in question.
Undatakah plays his death rattles in random order. (Lost my first legend endboss game when I thought he copied the order in which your guys died and so discovered a Whelp I absolutely didn't need to make.) Avoid Boom discovering deathrattle guys who give you a board if at all possible-- that Whelp ain't worth it.
Try to take a little health damage early to get an extra card off of Battle Rages.
You will have to dump some cards out of hand typically at the end of the game, so be thinking about that before you get to the end of the game. Remember, if you have minions that are going to hit the board there, you need to figure out how they can die before you can win. It's totally acceptable to Shield Slam your Acolyte of Pain or whatever. Boom HP can eventually save you if you mess this up but it's much better if instead you just don't mess this up.
If your kill is whirlwind/devastate then it's fine if you have one-health guys on your side, but if you're using Inner Rage then you need your board to be empty. This may mean you sometimes don't want to crank your Boompower to make minibots late, which will offend your sense of value, but hold off anyway. Basically, if you don't need extra guys who may cause problems for your kill, don't make them.
Do not play your second Devastate. Do not play your third Inner Rage/Whirlwind. You need those to win the game. This means you have to count a little bit but I believe in you.
Opponents will misplay against you a lot, although probably less if they've read this. This is a nice plus, enjoy it!
Special thanks to Ridiculous Hat for brewing assistance.
Questions? Comments? Concerns?