I've been researching to answer this for over an hour...
I'll add that if you've already fought this trainer, you can do a similar thing but instead obtain a Pokemon with a Special stat of 28, and fight a Ditto. Let it transform into you, and then either defeat it or run away.
It's also interestingly easier to get Blastoise than Squirtle or Wartortle, as their index numbers are 177 and 179