How do you get by that guy that wont let you go to the 8th gym?

I beleive you have done the Team Galatic's at the three lakes then and have let Mesprit, Azelf, and Uxie free from Cyrus, in Team Galatic Headquarters in Veilstone City, if not you must defeat Team Galatic at all three lakes and you must free Mesprit, Azelf, and Uxie from Team Galatic Headquarters, in Veilstone City. After you have done this, you must go up Mt. Cornet, by entering through the entrance near Hearthome City. Go up Mt. Coronet, defeating the challengers in your way, to the Spear Pillar. You will then face Team Galatic's Mars and Venus with your avatar's rival. After you defeat them you must face Team Galatic's Leader, and after you deafeat him, you must face Dialga. After you catch / defeat Dialga, the entrance to Sunyshore City will be open. Also, you will need Pokemon that have the moves; Rock Climb, Rock Smash, Surf, and Strengh, to successfully go up Mt. Coronet. * Note: I would bring serveral Revives and serveral various Potions. Hope this helps you!!!

Im Pretty Sure You Got To The LightHouse. Do You Know How To Make Cheats For Action Replay��

Defeat Team Galactic at the lakes. Then go to Route 216 (between Snowpoint City and Mt. Coronet) and explore Mt. Coronet. Hopefully you will eventually come across a little entrance that is hidden slightly. If it is the right place you will face Team Galactic as you venture further. On Pokémon Pearl you will see your friend. He will help you defeat Team Galactic's leader. Then you go through a doorway. You are then in Spear Pillar. Now you can either defeat Palkia or capture it. You only get to fight Palkia once. It is Lv. 50 and is WATER/DRAGON type. Your friend will not help with Palkia. When you've defeated or caught Palkia you will be allowed to go to Sunnyshore City and challenge Volkner (8th gym leader).