Easiest way to beat Cthulu is to beat him to the punch. Create your own demiplane, slow down time until you have 1000000 years passing by at a second of normal time, then pop back into the real world every so often to drop some Cursed Earth all over the place. You can spend your eternity in the Demi plane expanding it infinitely, meanwhile living long enough to research and know anything and everything. If he does show up and tries to cast dispel, you only need a caster level of 40 to beat it, and then you can shunt him away. Once he gets to your planet, sees everything burnt to s&&%, he'll just go home sad.
