I really need tips on how to beat the 3rd Gym Leader (that Ghost gym leader) because I can't beat it.

Do any of your Pokémon know a Dark or Ghost-type move? That'll help.

Have you considered receiving an Eevee from Bebe and evolving it into an Umbreon?

another way besides supereffective moves is just train ur pokemon with wild pokemon till theyre 7-10 levels above the gym leaders pokes
that works for me and i hope that helps

this is a real helpful thing.The easiest way tobeat Fantina is to use a Normal type pokemon. Normal Type pokemon are immune to ghost or dark attacks and they are immune to Normal so try and get a Normal type pokemon that you know has more than regular type moves and level it up plus Ghost and Dark moves are also super effective! That is basiclly the easiest but MAY need some time to level up or find a good pokemon the other option is not as smooth as the other but it is just to keep trying or levl you pokemon up! or try to beat Fantinas begging pokemon over and over again while ur pokemon level up and eventually you will get the the final pokemon and will somehow slide threw it and your welcome

I beat her with pure psychic pokemon. You can totally do this. Thinking about a good pokemon, Girafarig comes to mind. It's normal/psychic so it can use psychic to take out her Gengar and its normal type renders Ghost attacks useless. Although I despise them, dark pokemon would be an easy route to victory as well. If you don't feel like obtaining new pokemon, teaching your pokemon shadow ball (especially with a SPATT boost Move) can rip right through Fantina.

Thanks guys for the tips! I have beaten Fatina (Or whatever her name was)!