Hearthpwn

I've had tons of success with this list for Mecha'Thun Druid. It's styled as more of a control deck than a power-cycle deck, allowing it to have more success against aggressive decks than other faster variants.

Keep in mind it's not designed so that you have to win with Mecha'Thun. Sometimes you can pull out the victory using your minions alongside an attack-buffing Branching Paths, as well as the 3 damage hero power provided by Malfurion the Pestilent.

Your worst matchups are other OTK decks, such as Shudderwock Shaman or Mecha'Thun Priest, since you're a lot slower to pull off your combo, and you may be unable to kill off your own minions at times, however it's generally a fine trade-off as these decks aren't super popular at the moment. Quest Rogue can also be quite problematic.

If you want, you can replace Primordial Drake with a copy of Starfall however lately the drake has been more useful for me to close out games against aggro decks.