on dewford island. go by boat
Dewford Town.
you can find it near the second gym badge and its in a cave.
at the gym
every second gym leader will allow you to use higher level Pokemon
Go to the Pokemon league.
the ground gym
Destroys picachew
you travel to lavavine town
Pokemon league
Fortree city.
Beat the gym leader
it's in the woods between the first gym badge and 5th gym badge