This trick will only work if you have the Moon Pearl, but you do not need the third Pendant. At the top of the mountain in the Light World is a warp to the Dark World. Use this, then travel down to where the bridge would be. Squeeze very close to it, then use the Mirror to travel to the Light World. Do the lightest button-tap possible, then wait for the portal to bring you back. Do not move. Use the mirror again, and when you are in the Light World, walk into the fence. Facing the fence, charge your sword and slowly move back into the portal. You should now be on the edge of the cliff. Move right to jump off, then walk to where you see a lava break and a rock next to each other (at the bottom of the screen.) Move in between them and move down. The screen should move to you on a hill in the Dark World.