you need the skull fossil whic you can get underground
Go to serbii.net or cranidos serbii.net
a cranidos egg.
I can sure give you a cranidos for a shieldon if you like.
Cranidos
you'll get a cranidos in the under ground in Pokemon platinum. just keep on levelling it up till it learns headbutt.
You can get all the fossils underground, except for shieldon or cranidos. You find shieldon if your ID is even and Cranidos is your ID is odd.
lv.30
you can not find a wild rampardos in platinum you have to catch a cranidos by finding a skull fossil underground and then evolve cranidos at lv. 30 hope that helps =)
You can evolve Cranidos in Platinum and Diamond, but then you have to trade.
Must trade Cranidos appears in Diamond/Platinum (odd ID number) - Revive Skull Fossil
go underground get a skull fossil and get it restored
level it to level 30