Not that bad

Emperor 7 can be a little scary, but other than that most of the C'thun cards are just vanillas. If you're able to keep some removal for when C'thun drops you'll usually be fine. I used to get real salty about C'thun decks right when WoToG launched, but nowadays I'd much rather face a C'thun deck than any of the Tier 1 aggro decks.