I'd take pretty much anything over another "Ash-using-more-Pokemon-than-the-Leader" battle. I was very (unpleasantly) surprised when they went that route with Grant so soon after using it with Roxie. I get that it's a nod to the games, but Ash has a history of defeating MANY many trainers, Gym Leaders, and Frontier Brains using the same number of Pokemon as they did, no handicap required.
- Korrina could choose not to use her Lucario and go with her in-game team ("Lucario's off doing some special training so it can control itself as Mega Lucario", "Lucario's exhausted from all that Mega Evo training", idk)
- Korrina could use Lucario, but not Mega Evolve it because it still can't fully control itself as Mega Lucario
- Korrina could use Mega Lucario and Ash could manage a tie, a la Maylene
- Korrina could use Mega Lucario and Ash could somehow defeat it (which would be pretty unbelievable, based on what we've seen so far, but STILL preferable to Ash using extra Pokemon IMO)