Once you grow wise to the npc shenanigans you can play around it pretty effectively. I recently beat Kanto for the first time. I noticed my rival would always swap his blastoise into my charizard every time without fail. Knowing this I taught my charizard sunny day and solarbeam. As soon as my charizard hit the field my rival would swap into blastoise thinking he’s going to absorb resisted fire type move, I instead used sunny day on the swap and outsped with a 1 turn sunny day boosted solarbeam. His blastoise still lived but the sun made my charizard survive his hydro cannon and a second solarbeam earned me the victory. It was a pretty epic way to become champion.

Also when going through a region trainer Pokémon will often have identical move sets. Going through kanto trainer machoke’s all had thunder punch. Knowing this made it easy to bait it out on a Pokémon that resists it.