if the boulders are what i think, you have to have strength. sorry, i haven't done it before, so I'm not sure.:|
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Use Strength. It can be obtained at Olivine City in a building next to the Pokemon Center where there is a sailor inside. Teach your Pokemon Strength so that you can push aside big boulders.
use strength
You get it from a lady in vendanturf town, and use it in front of big boulders. you need the heat badge to use it out of battle.
Strength.
With the HM move strength.