I don't really see the point of this card. Aren't you usually trying to close out the game when you summon C'Thun? Besides, by the time you reach turn 10, I'd figure that your opponent would catch on that you're playing a C'thun deck and would save their removal for C'thun accordingly.
An interesting question - does your C'Thun really need taunt?
Usually he's going to clear the board or face damage for 10+ damage. Also usually, if your opponent survives he'll be eating some hard removal. Might help if you get to turn ten versus aggro, but at that point you've gotten to turn ten versus aggro and you're probably fine.