After clearing the Fire Temple, return to the Tower of Spirits. Go to the top of the spiral staircase and enter the room to start Floor 18.
First Tear of Light
The first Tear of Light is on the right side of the room. Use a couple of bombs to get rid of the blocks in the way, being careful not to alert the Phantom to your presence.
Second Tear of Light
After getting it, go to the upper left corner of the room, bombing blocks out of your way.
Pull the first block to the left, then pull it up when you can. Pull the second block to the left and down. Step on the switch to reveal a treasure chest in the upper right corner of the room. Make your way there and whip across to get a small key.
Take the key and unlock the door at the upper middle of this floor. Go up to Floor 19.
Go to the upper right corner and pick up the blue thing. Walk with it to the left side of the room, avoiding the miniblin on the armos statue. Put the blue thing onto the tile south of the switch, then rotate the blue thing to point left. Stand on the switch and use your boomerang to rotate the blue thing on the other side of the door until it points up. If the blue thing south of you isn't pointing left anymore, rotate it until it is. Then while still standing on the switch, shoot the blue thing south of you to activate the eye switch.
Go to the bridge that just appeared, running past the miniblins on armos statues. When you walk toward the left, a Phantom will appear and see you. It will roll like a boulder toward you. Just get out of the way. You can either make the Phantom run into the blocks around the second tear of light or you can bomb them out of the way.
Third Tear of Light
Use bombs or the phantom to get rid of the blocks at the bottom left corner of floor 19 to reach the stairs going up. Go up the stairs.
Make the Phantom see you to make it roll and break all of the blocks in this area. Get onto the platform on the left and use your boomerang to rotate the leftmost blue switch so that it is pointing upward. Move the block to the left of the platform with sand on it so you can hop over to the platform with the special tile. Pick up the blue thing that had been on top of the pile of blocks in the bottom right corner of the room and hop over the block to reach the special tile. Put the blue thing on the special tile, pointing downward.
Step on the switch and shoot the blue thing to the left. Go get the third tear of light.
The Wrecker Phantoms in this area can roll around like a boulder, as you saw earlier. Go downstairs after getting the Wrecker Phantom.
Have Zelda break through all of the blocks to the right, then have her go to the middle right of the room and get rid of all of the armos statues in the way. Then have her go around to the switch on the left side of the room that gives you access to the room with the blue arrow thing and the eye switch that you activated earlier. Have Link run in and get the blue thing out of that room.
Have Link take the blue thing that you just got and put it into the center area. Have him stand on the platform and rotate the uppermost blue thing to make it point at the eye. Then have him shoot the uppermost blue thing. It will reveal a chest that contains a random treasure.
Now go with Zelda down the stairs.
Your task now is to push the large and small blocks onto all of the switches in this center area. There is a block directly south of the stairs. Push it to the left until it hits the wall. Then have Zelda pull up the large block near the bottom-left switch. Have Link go through the gap you created and pull the small block down, then go up above and push the block down onto the switch.
Now have Link go up to the upper-left corner of the area and pull the small block to the left. Have Zelda push the large block onto the two switches in the upper-left corner.
Have Zelda go to the bottom-right corner and pull up the rightmost block, then push the one on the left to the right wall. Have Link push the small block right all the way, down all the way, and right all the way. Then have Zelda pull the lower block to the left, and have Link push the small block down on the switch.
Finally, have Zelda pull the large block in the upper-right down by at least one, then have Link pull the block from the left side of the room to the right and onto one of the upper-right corner switches. Have Link step onto the other. The spikes blocking the chest will be permanently lowered, so have Link run over and open the chest to get a small key.
Go back upstairs. Unlock the door to the right and go upstairs. Have Zelda roll up through the path of boulders and have Link run behind. Have Link hit the crystal switch to the left to stop the boulders.
When you go toward the lava to the left, Zelda will explain that she is unable to carry you. Have her roll down through the blocks below and go up the stairs.
Have Zelda roll around and break all of the blocks. Then the two of you should step on the switches in the middle of the room. Four stalfos will attack. Use the whip to easily break their bones and leave their skulls vulnerable. Then three Zora Warriors will appear. Use the whip to remove their shields. Then have Link kill the rat that appears from the hole up above.
Four Phantoms will appear at each corner of the room. Have Zelda fight them off so Link can run to the safe zone. Then when the Phantoms run over to the safe zone, have Link hit one in the back when it turns around, and switch Zelda into one of those phantoms. Then go downstairs.
Before tackling the puzzle on floor 19, go back downstairs to 18. Have Zelda walk to the blue arrow shooter in the top right corner. If she targets it, she will pick it up. Have her walk with the arrow shooter to the center area. Have Link step on the switch in the middle area and have Zelda walk onto the special tile in the room that opened. Have Link use the arrow shooter to shoot the eye switch in the room where Zelda is standing. Open the chest to receive a random treasure. To make the next part easier, switch Zelda into the Wrecker Phantom, then go back upstairs.
Roll through the boulders (or just slowly make your way up if Zelda is still a regular Phantom) and hit the crystal switch to stop the boulders. Then go upstairs to switch Zelda into one of the regular phantoms if she isn't already.
Have Zelda walk into the lava, then have Link jump onto her. Have Zelda walk back to the ledge where Link jumped from and walk up so Link can whip to the left. Have him pick up the arrow shooter and have Zelda walk over so that Link can jump onto her. Have Link put the arrow shooter onto the platform to the left of the rightmost eye switch on the platform up the stairs.
Now have Link whip back over to the platform where he got the blue thing. Have Zelda target the leftmost blue thing that is near the door, and have her take it and stand on the tiles south of Link. Have Link rotate Zelda's arrow shooter to point left. Then have Link shoot the shooter on the right to activate the wall switch, then have him shoot Zelda's shooter to activate the one in the room that opened.
Have Link and Zelda meet up to the right of the lava and have Link hit Zelda with something to get her to drop the shooter. Then have her walk into the lava and have Link jump onto her. Go to the door to the stairs that just opened, and go up. Then go around and go up the stairs.
Have Zelda walk up through the flames, then left and down and around the room to the boss key. She's safe in the spot where she picked up the key, so have Link run around and kill the Key Masters near her. Then have Link go with her to the upper left corner of the room and have Zelda walk to the right of the flames and stay in that spot. Have Link run around and kill the Key Masters in the upper right area with the bow and arrow. Then have Zelda go to the right, past where the Key Masters were that Link just killed, and have her step onto the red circle tile on the right. Have Link step onto the other red circle tile below. Then have Zelda go down and use the Boss Key on the big lock.
Go upstairs, have Zelda target one side of the door and have Link tap the other, then go upstairs and you will encounter Byrne.