The Leon vs Diantha fight is perhaps the worst moment in all of PM because no matter how you twist it there's no way for it to make any logical sense.
On its first portrayal and fight Alain was able to ride it out of G-Max with Chestnaught and while it defeated Chestnaught, who didn't go out without giving a good fight, for it then be handily defeated by Charizard X. It seemed like a normal enough Pokemon considering all, considering the stage. Strong, duh, it's a Champion Pokemon after all, but nothing crazy.
But then later on the gorilla is somehow Super Saiyan 4 Goku-tier. Based on the clipshow we got, it soloed FOUR out of Diantha's Pokemon, defeating even some of them with non-super effective moves. We also have to assume that Diantha either didn't set-up screens for reasons or that she did set up screens and the gorilla went and and swept through her entire team through screens anyway which just makes it even worse.
Some people after the Ash vs Leon fight are calling "Leon defeated Diantha by chipping at her entire team with Dragon Tail and then finishing them all with Rillaboom" but that's completely nuts. I seriously am supposed to believe that Diantha couldn't think of a way to deal with that with not just one but with FIVE Pokemon? Or that it took her so long to do so that the entirety of her team was at OHKO range for the gorilla? That Leon just stood there going DORAGON TAIRU LOL? That all of Diantha's Pokemon just stood there taking it? That she didn't yell a single order?
And again, based on the clipshow, only Hawlucha and Tyrantrum fought Dragapult, so this is just a massive assumption and stretch too. Trying to scram a square peg on a round hole.
We also have to assume that absolutely not a single of Diantha's Pokemon was able to damage it or wear it down in any sort of significant way too because of how fresh the gorilla still looked.
And then there's ****ing Fire Blast, Jesus Christ.
Any justification or attempt to make sense out of that battle is pointless.
With 95% of it being off-screened it is impossible to make reasonable speculation about what strategies Diantha used or what Leon did to counter them and vice versa that isn't "Diantha lost 50IQ points because of Leon's battoru aura".
I genuinely cannot comprehend how anyone can defend this "battle" in any way, shape or form.