First of all, get about 10 max repels and go to the summit of mount coronet where you would catch Dialga, and buy yourself a timer ball so you don't have to use your master ball, because you should use it on Arceus, Regigigas or Giratina, or one of the mystery gift pokemon. As I was saying, go to Spear Pillar where you catch Dialga and find the Adament orb (ups the power of Steel type and Dragon type) and then the blackout in Sunyshore stops and the guy blocking you really won't stop you know, considering the fact that you have a super awesome legendary on your team, but luckily he is gone, so you just go through the path to Sunyshore, but beware of the level 40 pokemon. Hope this helps, and the Sunyshore gym type is electric.
its for decoration
Go to the lighthouse(top floor) and talk to the guy wid blonde spiky hair
go to light house and find him
you have to catch dialga first
to get to sunny shore in Pokemon diamond you need to catch dialga at spear pillar
It is sunny in Sunyshore City.
Beat the Snowpoint City Gym leader, Candice, then defeat Team Rocket at one of the lakes, then he will let you past.
gym leader
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The 8th gym leader is in sunny shore city
at the sunny shore gym
he will appear at random in his grandparents house!