After you beat the elite four, go to the beach, south of lacunosa town. once you are there, you will see a rock that is partly touching the water, and partly touching the shore. Surf to the right, then up, so you go around the rock, then go back to land. from there, travel north, were you will see a set up steps, go up the steps, and follow the path until you see a cave (just before the cave on the path, you will see a boulder, i strongly recommend you use strength on that boulder, as it will provide a shortcut). once in the cave, you will notice it is shaped like a rectangle, go left, so you are traveling clockwise. once you reach the other end, you're there.