By battling Pokemon that you are comfortable with and winning them, but there is a choice, battling Pokemon that you are strong to, continuously. Or battle Pokemon you are weak to but can still kill them.
well, i level up pretty fast when the Pokemon you are trying to level up was a traded Pokemon, and when it has the lucky egg or exp share
level them up
It does not matter where you level it up.
Have the Pokemon hold a lucky egg.
Level it up to level 40
Level up at level 28
level him up to level 30
Get an Eevee and level it up in the Eterna Forest.
It elevates your Pokemon up one level.
level up your GOLBAT with high happiness.
Get an Eevee and level it up in the Eterna Forest.
If you want a good place to level up your pokemon, I highly recommend the Iron Island or the Fuego Ironworks.