Along with what Axelyx said, get a Slowbro or something else that can spread toxic and survive for a good while. I don’t remember my original team for beating the game but it had Ferrothorn, Slowbro, Talonflame, M-Lucario and Aerodactyl. I spent several hours checking all the battles and made a team which could use the different fields and be switched with TMs and Items to fill different roles, so for Heather I had M-Aero but then switched it to Lucario for some others, think outside the box and don’t be afraid to change items and the like in order to flex picks into different roles.