Hearthpwn

Do you enjoy playing literally DOZENS of mana worth of cards in a single turn? Do you enjoy watching your opponent squirm when they suddenly find themselves only being able to play one card at a time while you whirlwind through your deck? And do you enjoy watching them blow up, at full health, before they even realize what's happening? Then read on...

NOTE 1: Before anyone asks, yes Poison Seeds DOES proc Mecha'thun, despite the fact that it also summons treants after killing all minions.

NOTE 2: Be careful not to overload yourself with Lightning Bloom on the same turn as you play Celestial Alignment because you will be unable to do anything at all on the next turn.

Mulligan

Always look for Celestial Alignment as the deck relies on being able to cycle and dump cards as fast as possible. Nourish is also extremely valuable as a follow up to your Celestial turn. Against aggressive decks, Ferocious Howl, Branching Paths (armor option) and Spreading Plague can help you stay alive while waiting for your Celestial setup turn.

Play Phase 1  (Key Cards --> 1 x Celestial Alignment, ramp and card draw)

Your first goal is to play Celestial Alignment as soon as possible, with ramp and card draw ready to go on the next turn. Ideally, you want Nourish because it gives you mana immediately without giving any to your opponent. Do your best to ramp as fast as possible (Solar Eclipse is fantastic to couple with ramp cards) and start cycling through your deck. On a lucky run, you can sometimes manage to ramp up to 6-7 mana and burn through most of your deck on the first turn.

Play Phase 2  (Key Cards --> Twig of the World Tree, Sphere of Sapience, 1 x Celestial Alignment)

Between your initial ramping and Forest Warden Omu, you should easily be able to fly through your deck, grab a bunch of armor and block any incoming threats with Spreading Plague taunts within 1-3 turns from your first Celestial Alignment. Just continue liberally dumping any non-key cards and drawing until you're near (or at) the end of your deck, then put the Twig of the World Tree into play and drop the last Celestial Alignment to choke out your opponent again. On your next turn, play the Sphere of Sapience to activate the Twig's deathrattle to refill to 10 mana for Phase 3. 

Play Phase 3  (Key Cards --> Violet Illusionist, Mecha'thun, 1 x Poison Seeds)

The goal here is to dump everything else in your hand. The Violet Illusionist will prevent fatigue damage if you need to play out any remaining card draw spells. When you're ready, your last two cards should be Mecha'thun followed by Poison Seeds to proc the effect and kill your opponent.