Ash's mistake is his choice of party. Excuse me but two base starters? Lame. Kotetsu has a samurott, what the hell was Ash thinking to bring oshawott and snivy to a crucial battle? Hein???? Whatever if it's anime's usual promotion to anti-evolution to not, even Kotetsu was smart enough to fully evolve almost all his party (it will become fully evolved since riolu will evolve) and Ash chose a bunch of babies for cuteness basis? Excuse me, but if Ash would win this, it's nothing but armor plot. You guys ARE sure you want Ash to win with this? Seriously?
Looks at me Ash is not even taking the league seriously to not even use his stored pokemons which are like a thousands of times stronger. He would beat Kotetsu easily if he chose Sceptile, Infernape, Snorlax or whatever of any of his stronger and evolved pokés. But nooooooooo...
You guys forgive me, but that defeat will be deserved. Ash won't get to nowhere by thinking for sticking his party in their weakest forms just because the anime constantly thinks evolved forms are unworthy of a personality and bonds. I thought they got over that silly stereotype with DP thanks to Infernape's character development but no, exactly back to the square zero.