First he tells her to win a bunch of matches before looking for the lucariorite,
Actually, she won those matches on her own. She was just doing some training so decided to go for 100 straight wins and did that, we know she beat a Krookodile at some point according to her book. But Gurkinn didn't tell her to do that.
Then when she overcomes the trial against blaziken and she mega evolves lucario, it goes out of control and he scolds her for training him incorrectly.
He scolded her for taking their bond for granted, which was the source issue of the whole loss-of-control in the first place along with Lucario's warped Aura, not training him wrong.
Now, if he knew that Korrina trained lucario incorrectly, why did he let her evolve him? Why even bother putting her through that trial?
He didn't know anything, he only realized something was up after she Mega Evolved Lucario and it nearly killed Pikachu, the trial was because she was next in line to be Shalour Gym Leader and each Shalour Gym Leader has a Mega Evolution Pokemon partner. It's a tradition.
Finally, when Korrina 'learns how to battle correctly, she loses her very next match against Ash, who she defeated multiple times before. So when she battled her own way, she won. When she battled 'the right way,' her grandfather's way, she lost.
She lost that battle because Ash is destined to win every battle, whether he deserves the win are not. A large number of fans acknowledge the win as forced garbage so....eh...take it how you will. She didn't "learn how to battle correctly" but finally with Lucario and Maple's aid was able to get control of Lucario and understand their bond better and deepen it, which allowed Lucario to control itself.
That arc. slayed sooooo hard and it was engrossing from start to finish. Lucario/Korrina's characterization, growth, development, battles, character development, Mega Evolution overall entertaining aura and all was such a remember able part of it for me and I have to say it really showed what the writers are capable of with Korrina, so I never quite understood why the actual XY companions are so ignored. She truly was an Ensemble Darkhorse in the fandom and I really think her status was deserved. In my case, feel free to disagree, I care about character, especially female characters in this show showing me an authentic bond with Pokemon, constantly moving forward and battling in the Pokemon world and belonging. I guess that's why Korrina was such a savior for me and many others, she was flawed, she worked hard, she had been training since childhood, she developed, battled, she still had things to learn and places to go not just as a trainer or gym leader or Mega Evolution user but as a person as well. Yeah, she had to promote something(Lucario and dat Mega Evo merch along with Super Smash Bros
But that doesn't change the fact that she still shined hard. It's a strong example of how marketing doesn't have to do a character wrong, meanwhile XY had been marketing "Strong Shounen Hero, Ash" and "Cute, French Blonde, Serena" through Amourshipping and doing a pretty...uh unfavorable job with it from my POV. Korrina and how she got treated was something I wanted from Serena and Clemont above all else, consistently and from the beginning. She made mistakes, was fun, emotional, dynamic and totally invested in growing and those mistakes lead to consequences which required her to learn, that's Pokemon Anime written at its finest in a state for me. An endearing character had this brilliant arc and story deconstruction and came out a more founded, wiser, compelling and more balanced person and she didn’t change the substance, essence and heart of who she is and if you don’t think that someone with that much capability and agency over their story line, character and decisions isn’t phenomenal I don’t know what is. Her third episode was seriously the best thing of XY in my opinion, I was disappointed to her go. She would have fit right in as an MC. Oh well, there's always Mega Evolution Specials.