And his "strategy" is going to be convoluted convenient muck at best.
If he can't beat her through raw power, then he has to resort a lot to improvisation, attacks combinations and a bit of luck, either through abilities or missed attacks. If it isn't enough, he can train and retry later.
Remember that age and strength aren't everything in a match. Skills and tactics are very important as well.
Look for example at the battle between Clemont and his robot. Bunnelby had the disadvantage in terms of power and experience, and yet he managed to win thanks to Clemont's clever strategies and knowledge of his opponent. So pray tell... how come you find this battle good and even praise Clemont's strategy, while Ash's future battle (which has a similar setting, since Ash now has an idea of Korrina's and Lucario's tactics) would be unfair and glorify him?
Because even then Korrina has had this Pokemon for how long? Since she was a little girl and actively training it and working with it, being defeated by a Fletchinder he barely used, a newly-caught Hawlucha and relatively inactive Froakie is reaching. Pikachu couldn't touch the thing in its regular form and the Mega stuff is a whole different ballgame.
By that logic, Cilan's Pansage should be an overpowered juggernaut and be capable to destroy Mega Lucario easily, since Cilan got it when he was still a very young boy and trained it constantly for all those years. And yet, it has got its fair amount of losses though all its run.
Also, by that same logic, no beginner Trainer should even attempt to try to get Badges. I mean, how can you expect to see a young Trainer beat Viola's Pokemon with a beginner Fennekin, since even if it has an advantage it lacks years of experience? Arceus forbid for letting those beginners win against older Trainers, even if they train a lot specifically to do so and learn about their opponents' strategies!
I'll take a three on three against her regular Pokemon over him DEMing his way forward.
But wouldn't that make Ash pathetic, since Korrina has to "depower" herself in order to give him a chance to win? Using his four Pokemon to take out Mega Lucario isn't that unfeasible.
Even when not Mega Evolved, she was in perfect sync with Lucario and worked excellently with it.
Only up to a point, since Lucario tends to act of its own at times. So they aren't in perfect sync. Just almost, for now.
Lucario took out a Blaziken of all things and did in Pikachu like nothing, she clearly knows her stuff and she achieved 100 straight wins.
We don't even know against who she got the wins. If she considered even Team Rocket's Pokemon in her count, then some of those wins could have been done against less experienced people.
Obviously she's not stupid. She still has some to learn because her character isn't static, which is better than a character having nothing to learn and being praised regularly by everyone.
All people can make mistakes. Also, if you are clever enough, you can outsmart even an experienced opponent.
Flawless characters are boring anyway.
Ash isn't a flawless character. Yami Yugi from Yu-Gi-Oh! and Detective Conan
are flawless characters. Ash can't even be compared with them.
You may not like him, but that doesn't make him "perfect".