The gym leaders are psychic type trainers so the best type to use is a dark type Pokemon or ghost type Pokemon even a bug type Pokemon! Don't forget it will be a double battle so try using move combos.
beat the mosdeep city gym and talk to steven inside his house at the top left house on mosdeep
After you defeat the mosdeep city gym and the team aqua (also in mosdeep city but only after you finished the gym) you can get dive from the guy in the house in the south-west of mosdeep
have your back up Pokemon be like sabley or you can just train your Pokemon A LOT
you have 2 go to the seafloor cavern {south of mosdeep city} you have to beat the gym leaders of mosdeep though.then you use dive ans keep going til you see a submarine then you use dive again and beat everygrunt then you fight him.
Mosdeep City Surf there from Lilycove
mosdeep gym or from gardevoir
u get beldum after you beat the champion u go to his house in Mosdeep city near the seventh pokemon gym it is on his table just take it.
Tate & Liza in Mosdeep. Psychic type.
go to mosdeep city and it will be to the north of it
After beating Mosdeep City Gym.
on Mosdeep island ,steven gives you after difeating the gym leader
you get it after beating tate and liza mosdeep city gym leaders and after defeating team magma in space center. Go to steven house in mosdeep city he will give you dive.