We ranked all 10 Kanto gym leaders in Pokémon Red, Blue, ... Pokemon Origins Brock. ... having crushed the Fighting-type gym so badly with her Psychic-type Pokémon that it was deregistered.

She’s Pokémon's hardest Gym Leader for one very simple reason: Psychic was broken in Kanto in a way no other type has been before or since. This was especially true in the original Gen I games.