Hearthpwn

The gameplan is to use Plot Twist to finish the quest and search through your deck for the C'thun fragments.  At 10 mana, you can cast the fourth C'thun piece, Lorekeeper Polkelt, and Tour Guide in one turn, drawing C'Thun, the Shattered and setting his cost to 0.  From here, you can play C'thun or wait until next turn to combo it with Alexstrasza (also tutored by Polkelt).  The rest of the deck is healing and board control to stall until we can pull off the combo.

It's also worth noting that we can run soul fragments with zero risk of them bricking our hero power's cost reduction, as Polkelt will shuffle them to the bottom of the deck.  Additionally, a second tour guide in hand prior to playing Polkelt will allow us to draw either Twisting Nether or Soulciologist Malicia on the same turn we play Alexstrasza and C'thun, allowing us to remove minions from the enemy board that may otherwise soak up some of C'thun's damage.