The pokemon in the grass are all low leveled :Frown: would take so long to train. And we play the game kinda to ourselves so we have no friends to let use borrow the pokemon. Most people are not that trusting... But if that's only the real way...
I tried to train there but gave up eventually (it's too grindy) and just used RCs to get my Gengar up to 100, which allowed to me to beat the mayor's M. Gengar. His other Pokemons were considerably more manageable and I was able to finish him in my second attempt.