At Valor Lakefront to head on to route 222 to get to sunyshore city theres..

You need to have freed the three Pixies (Azelf, Mespirit and Uxie.) and also stopped Team Galactic at Mt.Coronet's Peak, Spear Pillar.

You have to go to the top of Mt. Coronet. I swear, no one pays attention to what anyone says in the game!!

Fly to Oreburgh City and stock up on these items:
5-10x Super/Max Repels
30x Dusk Balls
20x Full Restores
10x Revives
1x Escape Rope
^^You can never be too safe. Buy the Dusk Balls in Celestic Town or wherever you like. Also, make sure you have Pokemon in your team know HM03 Surf, HM04 Strength, HM07 Waterfall, and HM08 Rock Climb. Once you've done the above, heal your team [if necessary] and save your game.
Now, go north of Oreburgh onto Route 207 [I think] and go in Mt. Coronet. Start using your repels =] Use surf across the water and use rock climb up the rocks. In this room, try to go as north as you can. Once you've exited, you'll be on 1 out of 2 mountainsides. In the moutainsides, there are 2 caves to choose from, plus 1 cave that you came from. To continue forward, you'll have to go through one of the two caves. Don't worry, the wrong cave will just lead up to a dead end eventually. If you end up in the wrong cave, search around for the right one. There's a small part of Mt. Coronet connecting the two mountainside areas, but you just need to follow that to get to where you need to go... This second mountainside area is much more difficult and people often don't find it until someone tells them about it. Go up and use Rock Climb on the wall TO YOUR LEFT. And just follow that path and go in that cave. From here on-out, just follow the path until you make it to the end. Battle the two grunts, and heal your team [using revives/full restores]. Double battle the two admins with your rival. Once you win, your rival will automatically heal your team up for you. Battle and beat Cyrus. Once you win, and after the short mini-clip, DO NOT MOVE. HEAL UP YOUR TEAM!! Now, save your game! Continue the game once it is night in-game. ...When it's night in the game, continue your game. Battle Dialga and try to get his HP in the red zone and has some status problem, hopefully paralyzed. Start throwing dusk balls at him until you catch him, which really shouldn't be that hard. [Save your Master Ball for Cresselia, which can be caught later in the game. She's a pain to catch, take my word for it.] Afterwards, go to the back of this area to get an item [forgot what it was called] which can be useful for Dialga's sake. Now, go back into Mt. Coronet and use the Escape rope to exit. Fly to Pastoria, heal up, and you now have access to Sunnyshore city.

^^This took me a while to write. Please rate and verify it. Also note that for any mistakes you find in it, I did not use my game as a source or any walkthroughs online because I was too lazy. The game is also big, and I might have mixed up a few parts with totally different parts in the game. For that, I apologize. I copied and pasted that from a question that was very similar to this one. >.< It is not plaigarism, because I originally wrote this myself.

Head up to Spear Pillar and catch/kill/run from(first is preferred) Dialga and then he will move

When you have done the 3 lakes and the warehouse you need to go to MT. Coronet and after you have done this the person blocking your way will move out the way and you can go past

Hoped I helped *pk master*