You can't expect to beat a Water/Ice type Pokemon in the Elite 4 with a Water Pokemon of your own. Try catching a Magneton, Zapdos or maybe even Pikachu to kill it.
Blastoise is fine to use. Maybe not throughout the entire E4, but at least against Bruno.Blastioise? Blastiose is not good in the E4, Try using a Psychic pokemon, Electric, A Ground or a Grass type...