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In most games bosses aren't fought on equal terms. They want to kill you, you want to kill them, and you both do anything to succeed.
The majority of the boss fights in the Pokémon universe, on the other hand, are duels under official rules, and canonically they're on equal terms.
... except we get to know their next sendout and can switch accordingly, and we have a pack full of 99xFull Restores and revives. Not particularly fair.