this game is tough, and requires you to know about EVs and IVs at minimum.
Take some time to map out her movesets, set up the moves and EVs you want using the mochi guy in the market and the held item guy.
Formulate a plan for every pokemon you're up against. Treat it like an actual boss fight and not a pokemon fight. I forget if you can possess during boss fights but if you can, you get an instawin if you get them down to 1. Also, normally consumable items arent here, so you can cheese at least 1 mon with FEAR (focus sash, endeavor, quick attack, rat (or any other mon with endeavor+priority))