Shadao
Aim to be a Pokémon Master
^ This right here is the reason why I don't think Ash will win the league even if he proves himself capable enough to challenge E4/Champion level trainers. Look at how much he's been using Greninja just in this string of battles. First Alain, then Diantha, now Sawyer, and even Wulfric.
Ash-Greninja's power seems bottomless and unrivaled, but because of that, he's becoming way too reliant on it. The more he uses it, the more it's going to take on a toll on his own body. The premonition he had after battling Alain hints this as well. It's his only flaw in Kalos and I think it's the one that's going to come back to bite him. He'll best trainers like Tierno, Sawyer, and Alain in the League Conference only for him to collapse in the end against a trainer he would have no business losing to through overusing the Ash-Greninja form.
In fact, it's quite possible Sawyer could be the one to win the league depending on if Ash faces him before or after Alain. If it's after Alain, I can see Sawyer finally defeating Ash if he passes out from overusing Greninja while Sawyer goes on to win the league. It's just a matter of whether this will be in the Top-4 or if it will be in the final match. I for one, hope it's the final match just to reflect how far Ash has come in Kalos.
But here's the thing. That flaw aspect is already exploited and it's implied to be the reason why Ash has been constantly fainting when using this process. The culmination of this Achilles' Heel will not be in the Pokémon League, the Grand Finale, but in the 8th Gym Battle, before the darkest hour. Under normal story procedure, Ash would have to master the technique afterwards through humility and patience and then come back with a vengeance in his rematches against Wulfric, Alain, and even Sawyer. This is why I speculated and pushed forward for a 4-consecutive loss during his trials with Greninja, as it would be the most dramatic way to kick off that Ash and Greninja angst scene promised in the opening.