Maybe depends on what you have dark types are good against the leader.
Fortree Gym leader
Go to Mawille(eletric gym town) and there will be a gym leader... there is no gym leader
marshtomp
The gym leader of Fortree City's gym...Specializes in flying type Pokemon
flannery
Help me
Water Gym Leader
Go to Sootopolis.
Beat the gym leader
The Pokemon league!
juan
get better pokemon