You need to beat (catch or defeat) Dialga in Spear Pillar before going there. You'll need Surf, Rock Climb and Rock Smash/Strength (don't remember). Use Coronet cave between Hearthome and Oreburgh
You need to go into mt coronet west of hearthome city.
you need hm surf, rock smash, strengh and rock climb.this is to get you through.
you just have to follow it round and up to the top cave but past it is another cave go in there. that is the way to spear pillar. there you have to beat commander jupiter and mars.
then you go into the distortion world.thats where you have to beat cyrus.this is some information.
and after you have done that you should come out of turnback cave and you should be in sendoff spring. cynthia asks you to go see rowan. after you see rowan you should be able to get through that guy that blocks the route that leads to sunyshore city.
you should probably listen to DarthDestiny on this though because I think I just gave too much information about platinum. hope it helped anyway...!