The most common suggestions I've seen are pairing Blastiose with a nice stage 1 attacker. The most common suggestions seem to be Blissey, Cradaunt ex, and Gyarados.
I've got to tell you ... I put a fair amount of work into trying to make Blastoise/Blissey work and I never did like the results. I suspect that similar issues would arise with the other pokemon mentioned as well. First it was really slow going trying to get Blastoise, your attacker, and enough energy in hand to really make his power worthwhile. Secondly I never could fit enough of what I considered essential techs into the deck ... suff like SSU, Holon Circle, etc. It can be tricky to use 'Stoise's power without Circle so that's a must have. As for SSU ... it seemed to me that if you could ever save a badly hurt pokemon ... say a Blissey hit by Gallade's first attack ... with a ton of energy on it then you could just recycle that energy right back to your next attacker.
Anyway I never liked the speed, consistancy, or really power of what I built. I wish you luck ... because I really wanted this to work ... and hope that my experince can help you in some way.