This kind of problem could be solved easily with 2 options
a) Make the player choose the difficulty setting he prefers from the beginning
b) Make him choose the same amount of pkmn the gym leader has to battle him, like in the anime
Difficulties in this kind of game would completely destroy the social play balance, thus destroy the game itself. That's the problem with that notion.
a) Make the player choose the difficulty setting he prefers from the beginning
b) Make him choose the same amount of pkmn the gym leader has to battle him, like in the anime