you get a dark Pokemon for example mightyena
you get from the gym leader in mossdeep city NO YOU DONT I WENT THERE AND I BEAT THE TWINS I DIDNT GET DIVE
You have to beat the gym leader first... then you'll find them there.
You need Ghost-type pokemon, Dark-type, or maybe Bug-type pokemon. Don't use Fighting-type pokemon.
dark
Go to Mossdeep city and beat the gym.
you beat a cave and use dive in mossdeep city thats all!
After you beat leader Norman
Beat the gym leader
get better pokemon
get a pokemon, then you train it, get it really strong, and beat th ecrap out of all the 7th leader's pokemon. Its easy.
after you beat the mossdeep gym you have to go to the space centre and fight team magma with steven. after you beat them, you go to Stevens house in mossdeep city and he will give you TM 7, dive
with fighting type pokemon