Hate to break it to you, but the practical thing to do would be breeding a new Squirtle. Neutral natures are no good, and since you've already leveled it by battle (And, I assume, haven't been paying attention to EVs) there's no telling what EVs it has. Odds are that the DVs are nothing to call home about either. :\ Oh, and for future reference: don't bother posting the statline. Nature, moveset, trait, held item, species, and (optionally) EVs/DVs are sufficient. :3
At any rate, Blastoise:
Blastoise @ Leftovers
Bold Nature (+Def, -Atk)
EVs: 252 HP / 220 Def / 36 Spd
Surf
Ice Beam/Toxic
Rest
Sleep Talk
This is probably one of the simplest sets to use. Just tank out with Blastoise's high HP and Def, healing away damage with Rest while continuing the onslaught with Sleep Talk. Toxic hits bulky waters, but Ice is good against Grass types and the like. If the STalk combo isn't your cup of tea, though, you could always try something with Blastoise's plethora of support moves. Rapid Spin, Yawn, Protect and Haze are all fairly viable options and Mirror Coat might be a fun surprise against random Thunderbolts or Grass Knots and Counter can do the same against Thunderpunches. Whatever the case, Bold nature with Surf and Ice Beam (or Toxic) is usually a good place to start.
