First you need an action replay.Then activate the walk through walls code and walk through victory road to route 224. Keep going up until you see a large grey rock.Walk about 4 steps right then walk up across the water.There should be a small stretch of land,walk on it when you cycle to the end shaymin battles you.

Just thought I'd show you my pearl team :)

Infernape
shiny scizor
shiny garchomp
palkia
lucario
luxray

(all over lvl 65)