you have to go through Mount coronet the route 208 way
hope this helps
at the top of mt coronet(arch=spear pillar)
spear pillar
Spear Pillar is exactly where it was in Diamond and Pearl; Mt. Coronet.
in spear pillar in the middle of mt. coronet
the top of mt cornet is spear pillar
If you keep going through Mt Coronet you'll eventually reach Spear Pillar, where you catch Dialga, Palkia and Arceus. You can't fly from Spear Pillar, though.
Dialga is in Spear Pillar and Spear Pillar is located deep in Mt. Coronet.
You go to mt. coronet from harthome city.
in the very middle
in spear pillar after the snowpoint gym challenge (spear pillar is in mt coronet)
Spear Pillar, which is at the very top.
You will find them in the spear pillar.