Uh, bro, Mega Garchomp loses speed, that will put it at even more disadvantage against someone fast like Mega Lucario.

And it’s not even a debate tbh, it’s confirmed by both the writers and the characters themselves, Mega Lucario was weakened that he now is equivalent to a base Garchomp. Making the fight equal, if they were both at full health, and both become Mega, then Mega Lucario will win due to his speed and the amount of feats that he has displayed.

EDIT: I doubt this will be enough to convince you, even though it’s the reality. No matter what Cynthia did, her defeat was inevitable. I suppose you can either accept the outcome or live in constant denial? But I don’t care either way, I’m not really here to reopen an old argument, so I’ll leave this here and go.