Posting this just after beating Charlotte myself,
The MVP of my team against Charlotte was hilariously my Ninetales. I didn't expect it to be more than a huge fire sponge, but in the end, Maxed Flash fire + Sunny day + Overheated/Burning field was such a boost that it hardly matters that Charlotte's team was also fire-type anymore. The flash Fire also made it so that Ninetales was pretty much never damaged during the fight, as Charlotte's focus was seemingly always either focusing on my other pokemon or using fire attacks that hit both my pokemon.