Once you leave the Dark Palace (and replenish you medicine in the Light World), head south from the hills to the bridge. With the hammer, you can now knock down the stakes that are blocking your way. Continue south until you get to the lake, dive in, and swim to the shallow end surrounded by stones. Use the Mirror to return to the Light World and you'll be on an island with a Heart Piece.
Go back through the gate, swim east until you get in front of the Ice Palace and use the mirror again. In the island cave you'll be in front of is the Pond of Happiness. For every 100 rupees you'll throw in, you can increase your capacity of bombs or arrows. Return here every time you have money to spare until you've reached the maximum capacity.
Now head for the Swamp in the Dark World, to the west and south of the bridge. The shrine here is the Level Two entrance, but you can't reach the ledge on the left. Remember what you did with the floodgate in the Light World? Warp back to the Light World shrine and open the floodgate again. When you return to the Dark World, the shrine there will be flooded and you can reach the ledge.
Once you're on the ledge, take out all the water monsters to reveal a chest with a key to open the door downstairs. There's another key under the farthest skull in the next room, Bomb the wall in the top corridor to get to the Map. Now open the door in the previous room's south corridor.
In the next room, go through the open north door to find another key under a skull. Use it to open the locked door. Use the Hammer to get to the switch in the north room and push it to open the floodgate. You can now swim to the far left ledge in the south room.
In the big room with the shallow water, take the south door and follow the loop starting to the right. When you come around to the blocks, push the bottom one to clear your way and get the Compass.
Now go back to the big room and take the southwest door and follow the same type of path you did for the last key to get another. Use it to open the northwest door in the shallow water room. Use the boomerang on the crystal to get to the switch to push it, allowing you to get to the west ledge in the south room. (Be sure hit the crystal again to reset the blocks before leaving - trust us, you'll need it.)
In the next room you can push the blocks to get through to the staircase. In the upstairs room, you can replenish your hearts by lifting the skulls in the south end. There are four pits here - you want to jump down the northeast one (push the block to get to it) and head east to find the Big Key. (If you didn't hit the crystal a second time earlier, you can't get to it.) Now return all the way to that first big room with the shallow water to open the big chest and get the Hookshot.
Use the Hookshot on the skull to get to the right hand ledge and get a key under the farthest skull. Now grapple your way to the north ledge from the center and open the door.
In the next room, lift up the skull in the northwest corner and push the statue on top of the switch to open the shutters. Take the north-EAST door, then follow the doorways and stairs until you reach another floodgate switch and push it to drain the water. Once you can go through the door, take the north doorway to the room with all the waterfalls. Walk through the second waterfall from the right and follow the path downstairs to the large pool.
Swim against the current to get the key under the skull at the northeast end, then open the door at the northwest end. The path is now clear to the Boss.
The Hookshot is the secret to beating Arrghus. Use it to grab the orbs around him and slash them before they float back. (Stay out of the way when they shoot out in a ring.) Once Arrghus attacks on his own, power up your spin attack when he leaps in the air and release it when he comes down and approaches you. Once he's gone, you'll rescue the second maiden, who will give you hints about other Dark World gates, and the identity of who's really behind all this.
