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I managed to beat Koga with X item spam which honestly is probably the best chance I have given how I'm basically stuck being 10 levels underleveled for the rest of the game. I think the only other mandatory trainers are Blue, Giovanni, the other Gym Leaders, Victory Road trainers, and possibly some gym trainers. I don't feel comfortable trying to fight Blaine or Sabrina at this point - I think my only option is trying to deal with Blue and hopefully evolve Wartortle and beat Blaine with more X item abuse.
Does anyone have any other suggestions?