also, what is it with his bland personality? he's just a mary sue who got to the league and is praised by people because he 'got to the league' a lot faster than ash. time skip...and he doesn't even use a lot of strategy. just a lot of...*sends out pokemon* Return, Jolteon or Scyther! his movesets are pretty bad imo, ash does a better job with it.
greninja wins for a few reasons. one, ash helps it have better reaction speed because he can notice things that greninja can't and with these improved instincts, red's charizard isn't going to go well. if he's using turn based system, ash will wreck him so hard with dodging and water shuriken. yeah, mega evolution does give him a boost, but fire spin? seriously? how will that help? slash is a terrible move to go up against greninja who has double team to help it avoid that move and then follow up with a counter, plus greninja excels with close range combat as that's it's battle style. Swift can counter this, yes, but I highly doubt that Ash will just let that slide and will counter rather quickly. ash greninja has also been able to go toe to toe with diantha's mega gardevoir, who is no undoubtedly stronger than any of blue's Pokemon.