In Day 5, after you've solved the stone puzzle and visited Oba's bike shop, you can head southwest through the wilderness area (to get back to Avalons Peak) before going to the Quarry. Once you get to Avalons Peak, head south and enter the Golf Course area.
Head to the right and you'll see the building from earlier in the game where the girls shattered the glass door with the "closed" sign. Select Demelza and charge her gauntlet ability and then move across the shattered glass to the door with the "closed" sign. Go inside, and then head right to the other door. Once outside again, head to the north east corner and there will be a small gate, with a generator next to it, tucked away in the rocks. Use the gauntlet to power the generator and the gate will open. Follow to the path to the Pyramid structure and interact with the stone underneath it. After some short dialogue, the achievement will pop.
Feel free to let me know if I can improve this solution in any way.