you have to go to the top of mt. cornet to get to spear pillar and you getsucked into Gratiana's lair distortion world) its some sort of puzzle.
you beat the gym leader
go to lighthouse (top) talk to gym leader
in the grass in the last city the 8th gym leader
from that gym leader on the beach of sunyshore city (must have beaten the 8th gym)
In the lower right side of Sunnyshore city
Wallace is the 8th gym leader PEOPLE!!!!!!!!!!!
The first gym in Pokemon diamond is in Oreburgh City
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.
The Pokémon the second Gym Leader has in "Diamond" are Cherubi, Turtwig and Roserade.
8th gym leader
The 6th gym leader is in Canalave City.
The 8th gym leader is Drayden he uses Dragon type pokemon