I'm going through the post-game content in White 2. I want to complete the Pokédex, so I want to spend more time in some areas and catch all the available Pokémon. So I need low-level Pokémon to get those low-level Pokémon down to low health without killing them. So I have a low-level Koffing and a low-level Basculin to poison and do damage to other low-health Pokémon without killing them, so I can catch them.

However, there are also trainers that are even tougher than the champion, so I need my actual high-level team at full health to defeat them.

Am I supposed to be going back-and-forth to the Pokécenter and swapping Pokémon out and in, or is there some strategy I'm not aware of?