you need to use a strong fighting type like a lucario or medicham
After you beat the sixth gym leader, you are allowed to use strength which moves the boulders.
go to the seventh leader. -_-
After you beat/catch garintina in diamond you could keep training or just keep fighting the pokemon league. Also, you could go catch more legandary pokemons.
beat the #?*@! gym leader
what did you say
just beat the fourth gym leader
you beat the gym leader
beat the 2 gym leader
Beat the gym leader then visit the Library
the best way to beat the 7th gym leader is with a fire type Pokemon
jubilife city i think
It's the gym leader that allows you to use strength after you beat him.