Well Kiawe may stand a chance if numbers are restricted (eg. a 3 on 3 battle). His Charizard would likely feel like a downgrade from Alain's Mega one however, but Ash doesn't really have Greninja level power here anyway.
I have noticed a games character has never defeated Ash in the league however. In fact I think Gary and the XY ragtags were the only games-originated league competitors so far.
On a side note, after letting them get some dangerous Pokemon and the series' gimmick for once, I really want at least one solid Team Rocket battle this series. Like no curb stomps or plot armour power boosts to contrive the twerps winning effortlessly, just one on 'Do I Hear A Ralts' or BW level where both sides get in some impressive hits and counters, and it really feels like the twerps sweated for their win for once. I feel like that would be difficult with TRio's limited Pokemon number compared to double the amount of companions this series however, unless by some miracle they actually make Bewear and Stufful battle for them.
The nearest to a non-one sided TR faceoff this series was the Alolan Meowth one and the mech battle with Misty, where TR still weren't really landing any offence, but it felt like the twerps were being forced to think up a lot of counter attacks against them. Surely we can up that a LITTLE at least, even DP managed to do a bit better.