Lucario basically stopping Charizards GMAX before going down allowing Pikachu to win will be what Ash failed to do in XY and will be the strongest feat of any Ash Pokemon because Charizard GMAX is the most powerful Pokémon in the world
G-Max is not like Mega Evolution.
It's a temporary power that only lasts 3 turns. We've seen lots of regular pokemon be able to hold their own against G-Max Pokemon/G-Max attacks.
The anime seems to follow the games by allowing only 3 G-Max attacks. So tanking/avoid 3 attacks is not a big deal. Perfect example is the Rilaboom versus Chesnaught fight.
Unless of course, they make Leon's G-Max Charizard special and reveal that it is capable of lasting in its G-Max form much stronger (Given that it's Leon's Pokemon after all).
Would be better if Pikachu tanks/evades Charizard's G-Max moves and finally takes it out with 10MV.
Plus, if Ash is going to need multiple pokemon to beat Leon's Charizard, that would narratively be weak as this would mean that Leon could simply challenge Ash next season and take back the title in a 1v1 battle.
Obviously next to no chance of happening. However, in an ideal world, Ash goes back to train with Greninja Pre Lucario fight. He runs into Diantha and Alain there who offer to take care of the vine situation and Greninja rejoins his team.
AG is the final ace against Leon's Charizard.