Instructions
Travel to Blackthorn City and enter the Pokemon gym located there. The gym will be a large building with a sign in front reading "Blackthorn Gym."
Once in the gym, go up the stairs and walk north. You will see two rocks. Push the rock on the right up two spaces, and push the rock on the left down two spaces into the nearby pit.
Walk down the next flight of stairs you see to the first floor, then find another flight of stairs in that room to return to the second floor. You will see two more rocks. Push the right one down three spaces and the left one up nine spaces, then right two spaces into the pit.
Walk north on the second floor of the gym. You will see two more boulders. Push the one next to you right three spaces. Push the other boulder in the room down one space into the hole.
Head to the staircase you saw in Step 2 and go down the stairs. Start moving north once you are on the first floor of the gym. You will eventually come to a boulder path, which has been created by the boulders you have been pushing into holes. Cross that bridge to find Clair, the last gym leader.