You need the Cobble Badge.
Well any pokemon that can use the strength tm to move that boulder out of the way unless it's a big boulder you cant move that
the most probable mistake is that you did not beat the gym.
you cant be a Pokemon in Pokemon diamond
you cant you cant
This boulder is probarly a boulder you need the HM strenght for, which eenables a pokemon to move bulders.
you cant get Lugia in Pokemon Diamond
You cant get to hoenn on pokemon diamond. Sorry.
you cant
you cant get that Pokemon.
you cant get it on diamond or pearl!
You cant
you cant