Well, what level are your Pokémon? Sometimes you can just brute force it if you don't have anything good against the Gym Leader! But I'd focus on getting at least one of every type so you can build a balanced team!
[Ryuunosuke, what are you talking about you only raise Ghosts and Dark-types.]
text;
[Ryuunosuke, what are you talking about you only raise Ghosts and Dark-types.]