>>57541333She played it wrong. She thought he only two options are
1. only use shadow Pokémon to make the purify all the Pokémon task as efficient as possible
2. Use a team of purified Pokémon but stop playing the game after the credits and leave dozens of Pokémon unpurified
When anyone sane would just play with a team of purified Pokémon then use their postgame resources to make the purify every Pokémon grind as fast as possible. Hell you could use a main team of 5 Pokémon and work the shadow gaugea down throughout the latter half of the story one Pokémon at a time?