if you have a lot of rare candies level your Pokemon's until its level is bigger than the gym leaders.hope that helped you!
beat the 8th gym
Blue (The eighth gym leader) is waiting on Cinnabar. He won't battle you unless you've beat the rest of the gym leaders
Gym Leader Blue
You have to go into the Dragon's Den behind the Gym, there you must find the Dragon's fang.
beat gym leader
Go to the third
you go to blackthorne city and challenge the 8th gym there
Simply beat the gym leader in every gym.
8th gym leader
you go west of ecturek and face the gym leader of olivine
defeat all the other gym leaders
you beat the last gym leader in johnto