Instructions
Travel to Lilycove City, located east of Route 121 and Fortree City.
Exit Lilycove City, using the western exit to reach Route 121.
Travel west on Route 121 until you reach an intersection. Travel south at this intersection to locate a small dock.
Use the Pokemon move "Surf" to enter the ocean at the edge of the dock.
Surf east, followed by south when you reach the large rocks blocking your path, followed by west when large rocks block your path south. Immediately upon surfing west is the entrance to Mt. Pyre.
Surf onto the small patch of land in front of the cave entrance. Walk into the cave opening to enter Mt. Pyre.