i suggest, just raise your Pokemon
dark
Battle the rest of them
Beat the elite four again but they get stronger
in the survival area [after u beat heatran]
all sinnoh pokemon tips:just battle all wild pokemon.trainers,gymleaders,and elite4
you get a dark Pokemon for example mightyena
Go to Mossdeep city and beat the gym.
Steven gives it to you in mossdeep city. You have to beat the mossdeep gym before you can use it. They are Psychic types so be careful.
you will have to beat all the gymleaders
arceus will beat him easily.
You must beat the MossDeep City gym to use the HM for Dive.
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