You can stat out Cthulu in the same manner as any of the other gods, except Cthulu would have a much higher divine rank.
Not really, he might be a god but nothing lasts forever in the lovecraftin universe, and he cant claim to be the oldest, wisest, or strongest of the gods. Heck, he cant even leave his own tomb unless someone else helps him. I would say that he would have a very hight divine rank, but not neccisarily be above everyone else.

As a side note: If i want to take some folks who think they can take on anything and give them Cthuhlu to test out, i will. I just want it done right, and i cant make monsters very well.