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You must use Blastoise against fire, soil and Rock opponents. If you're worried that Hydro Pump is not reliable enough, teach HM 03 - Surf. May you also want to add non-moving water, like Blizzard or Dig Earthquake and to give it a wider range. When confronted with a fast, the high-level opponent, it's a good idea to slow down first (for example, using an electric Pokemon equipped Thunder Wave), then follow up with the devastating attacks Blastoise. Blastoise's Achilles heel is obviously the weakness of its electric attacks. So if you are against a powerful Blastoise, to use a fast Electric type like Jolteon or Voltorb or type of grass, if you have not the last.