Final Fantasy IV/Tower of Zot to the Underground

Tower of Zot
In the room where you awaken, go to the left then all the way to the top of the room (northeast corner). Go through the door you find there. In this room simply go as far south as you can, then as far west. Then go up to find the door.

B3F
In this room, take the bottom path and go as far to the east as possible. Then you will head north, and then west again, and you will come the next door. In this room, you will head to the south, and then as far east as possible. Now head north and go through the door.

Save Point and Magus Sisters
The first door is a save point. After saving, go toward the next door. You will be interrupted by the Magus Sisters. The key to defeating this enemy is that the one in the middle is the one who can heal the others. So you must destroy the one in the middle first. Also, they cast Wall on the middle one so, spells will be reflected. The easiest way may be to cast spell on your own party, and reflect spells. Once you defeat the middle one, attack either of the others until they are defeated.

Tellah Fights Golbez
You should return to the save point because you will have to fight in the next room. First, a cut scene will begin in which Tellah fights Golbez. Tellah dies from the battle. Go speak to Kain and he will join you as does Rosa when you save her. Turn around and leave this room, but before you can leave, Valvalis will fight you.

Valvalis
When she starts to spin around you must have Kain jump. Otherwise just have the rest of your party attack, and have Rosa heal. Once you defeat her Rosa will warp you back to your room in Baron. Kain will give you the Magma key so that you can reach the underground.

Go Underground
Exit Baron castle and the airship will be outside. Get in and fly directly south of Baron. You will come upon an island with a cluster of mountains and a town. Go into the town. Walk into the middle of the town to the well. When you approach the well, choose the Magma key and it will fall in, opening up the underground. Fly in with the airship.

Dwarf Castle
When you fly into the Underground, a cut scene will begin and you will be attacked by the Red Wings. Once it is over you will be right next to the Dwarf Castle. Keep going straight ahead until you reach the throne room. Speak with the king, Cid will leave your party and you will see dolls bouncing in the background. The king will have you check it out.

CalBrena
If you are able to kill all of the little dolls quick enough, the battle will be simple. Otherwise you will have to fight the large CalBrena. Be sure to fight this quickly, because otherwise, she will split apart again and you will start the cycle over again. After defeating the dolls you will be led directly into another battle.

Golbez
Golbez will attack you to the point of near death, but Rydia will come in and save you. Quickly revive one of your party, and have Rydia use a call magic (Titan is pretty effective). Golbez will be defeated fairly quickly if you survive long enough. If Kain is airborne (from Jumping) when Golbez uses Hold Gas, he won't be affected or killed with Rosa and Yang, which lets you fight with three people instead of just Cecil and Rydia. Once you defeat him, the king will open up the underground passage for you to go after the next crystal.

Note: If you cast Warp in the Throne room, you will return to the secret crystal room where you can collect the Dark Crystal. This crystal identical to the crystal found in the Sealed Cave, and allows you to skip past that entire area.