After you beat the canalve city boss you go the library and go to the top floor there u will find the professor and friends he will talk to you and tell you to do stuff and eventually after you have gone to all the lakes and done the stuff you need to there you go to the galatic HQ in Vielstone city you beat the boss and then follow him to the top of MT. Cornet and battle Dialga (PS you can catch him with the masterball) go to Valor lakefront and now the dude that was blocking the path out of the right of town is gone! follow that path until you come to a city this is Sunyshore City where the 8th Gym is.
in the grass in the last city the 8th gym leader
In the lower right side of Sunnyshore city
The first gym in Pokemon diamond is in Oreburgh City
The 8th gym is in Sunyshore, but you will be unable to go there until after you beat Cyrus on Mt. Coronet.
After you defeat the 8th Gym leader you can get HM08 from the Jhoto person standing on the Sunyshore beach.
you beat the gym leader
Nope
Sunnyshore City Gym: Electric (beacon badge from Volkner)
from that gym leader on the beach of sunyshore city (must have beaten the 8th gym)
in the grass in the last city the 8th gym leader
In the lower right side of Sunnyshore city
go to lighthouse (top) talk to gym leader
the fifth gym on Pokemon diamond is the one in herthome city,
The first gym in Pokemon diamond is in Oreburgh City
you will get it from the girl at the 8th badge gym she will give you the hm 07 waterfall
The 8th Gym Leader has electric type pokemon. Your best bet to defeat him is Ground or Rock pokemon. Electric types are not effective against ground types.
you have to catch diagla on top of mt coronet than you should be able to