Yep, Greninja is certainly very strong and one of Ash's strongest Pokemon, but not nearly as strong as some people claim. It dealed many hits on Diantha's Gardevoir, sure, but based on circumstances I seriously doubt Diantha fought with full strenght. If it would be Champion level Pokemon, it wouldn't have lost to Alain's Charizard that way (as I said, I think Charizard still had quite a lot of strenght left when it defeated Greninja). I also think if Charizard would mega-evolve, it would easily surpass Ash-Greninja, and I don't even think it's impossible that even now Charizard would be stronger than Ash-Greninja. It has been so long since Ash's Charizard battled at full strength (thanks to XY skipping Ash's Charizard mega-evolving and Charizard's lack of focus in BW) that we don't know Charizard's full potential at the moment. I'm sure it's these days a lot stronger than it was when it defeated Noland's Articuno, though.
By the way, I am now studying theology at University of Helsinki (Helsinki is a city in Finland where I live), and I still want to become a priest
By the way, I am now studying theology at University of Helsinki (Helsinki is a city in Finland where I live), and I still want to become a priest