i beat that gym with pokemon in their high 20s but i had a full team at that point, ideally you want your pokmon to be mid level 30s.
canalave city
byrons gym at canalave city. You have to use surf at route 218(west of Jubilife City) and then you can get in canalave city.
canalave city
Go to the lake south of Veilstone City.
Go to Canalave City.
You need to head to Canalave City via Jublife City.
You get stregth after you beat the canalave city gym leader
beat the heart home gym
you need tough pokemon that can beat him
to help the boy in canalave city, you have to go talk to the sailor and beat cresselia and then after you beat him go back into the sailors house with the boy in it and give him the feather thing...
byrons gym at canalave city.
on crescent moon island in canalave city after you beat the game