1. Ramp mana && survive early game with removals
Wild Growth && Biology Project are at your disposal
2. Draw your combo pieces as you go
There is a Witchwood Piper to help you pull your Wild Pyromancer or Flobbidinous Floop
3. As soon as you have both Flobbidinous Floop and Mecha'thun and situation allows - play Mecha'thun
You do not care about the condition yet, it's just to make Flobbidinous Floop remember your Mecha'thun. From this point on you can no longer play any other minions - spells only.
4. Draw remaining combo pieces, namely - Wild Pyromancer, Living Roots, Astral Communion
5. Draw your deck
6. OTK combo
- use Flobbidinous Floop to summon 3/4 Mecha'thun, 6 mana left
- Coin/Innervate/Biology Project to get an extra full mana crystal, 7 mana left
- Living Roots on Flobbidinous Floop making it 2 health, 6 mana left
- drop Wild Pyromancer, 4 mana left
- play Astral Communion discarding what left of your hand, Wild Pyromancer will deal 1 damage to all minions on board, 0 mana left
- Astral Communion generates 0 cost Excess Mana, play it
- both Wild Pyromancer && Mecha'thun die, leaving you with no cards in hand and you managed to empty your deck before...
There are some additional challenges to overcome - you should not have ANY minions with more than 2 health on your side of board when doing the OTK
- initial 10/10 Mecha'thun must die... But I don't think that's gonna be a problem... after all it's 10/10 and it's good for smashing your opponent's face - just 3 hits and you don't need an OTK :D
- remnants of Spreading Plague technically could be issue as well...
Only real games will show if it's gonna be a significant problem... This is why I think Warlock's version of combo with Cataclysm is going to be so much easier and consistent and with introduction of Galvanizer it's viable even in standard...
Interesting times are coming :P