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Reposting my objectively superior retelling of the fight
>introduce Alain earlier
>he and ash reminisce about everything that happened in XY
>they promise to meet in the tournament for one last rematch
>Alain gets a full 6v6 with Leon
>he prepares well ahead of time, pulling out tactic after tactic that barely work and cost Alain pokemon
>Alain is clearly crestfallen as the match is now 3 vs 1 pokemon, Alain only has his ace left
>Leon notices and throws out his zard in respect to Alain's efforts
>Alains mega evolves but is clearly struggling against even Leon's base zard
>Alain realizes he can't hack it and almost outright surrenders
>ash suddenly yells from the stands for him to do his best
>quick flashback to Greninja vs Megazard
>Alain gets a second wind and knocks Leon's zard down for the first time in a head on clash
>Leon smiles and realizes he won't be able to take it easy anymore
>gigas his zard and finishes Alain
>Alain is obviously upset but holds his head high and tells ash to beat him
There. Simple, doesn't need too much setup. Has just enough emotional payoff. Put greninja in the stands of the fight too if you want to be extra cheesy.