Treasure: 10 Gil, 155 Gil, 750 Gil, 795 Gil, 880 Gil, 1250 Gil, 1455 Gil, 1520 Gil, 1520 Gil, 1760 Gil, 1760 Gil, 1975 Gil, 2000 Gil, 2750 Gil, 4150 Gil, 7340 Gil, Potion x2, Antidote x2, Gold Needle, Tent, Cottage, Staff, Claymore, Mythril Axe, Ice Brand, Mythril Helm x2, Flame Mail, Mythril Shield, Flame Shield, Mythril Gloves, Canoe.
Crescent Lake
Crescent Lake has a new selection of weapons and armour, as well as level 6 magic for your White, Black, and Red Mages. It's expensive, but you should check them out anyway. After you're done outfitting your characters, go past the Black Magic shop and follow the path through the forest until you reach a group of sages. Talk to them and they'll tell you some important stuff about the remaining three fiends. Most of the other sages will tell you the same stuff over and over again, but one of them will give you the canoe, which you'll need to get to Mt. Gulg.
Exit the town, bust out your newly acquired canoe and up the river. There are several annoying enemies you can encounter here, I would suggest you run away from most of them unless you have a cabin on hand so you can refill your magic charges. Either way, if you choose to attack them, do it with your Ice or Bolt spells. Jump into your canoe and head left until the river branches off to the north, follow it up. Take a right at the next branch and continue up, then take a left at the next branch. Continue up and take a final left at the branch. Now all you have to do is follow the river and you'll near Mt. Gulg. Cross the river with your canoe and use a tent or a cabin if you used some magic beforehand.
Mt. Gulg
Floor 1
The first floor is just a very large circular room with lava practically everywhere, save the few little dots of safe ground here and there. There's no treasure here, and more importantly, no enemies that will give you any real trouble. The obvious path to the next floor would take you down around the bottom, then back to the top around the left side of the floor. An easier choice might be to just cross the lava field to the left of the entrance to Mt. Gulg. You'll take some damage but generally no more than you would if you took the first path.
Floor 2
Floor 2 is, in a nutshell, a long corridor connected to a very large room which coincidently has a large cache of treasure, but almost all of it is guarded by some rather annoying enemies, one of them are Pyro, and the other are Pyrolisks. Pyros are tough, but weak against Ice spells, so use them and they'll die rather quickly. Pyrolisks are nothing without a group as they are "pack hunters" so to speak. They are pretty weak but they can use the instant-kill spell Glance, often, and when you're dealing with a group of Pyrolisks, this means trouble. Use a strong spell that can hit multiple targets to get rid of them easily. Once you've collected the treasure, the stairs to floor 3 are at the far end of the aforementioned corridor, which you can reach via the two doors near the far bottom left corner of the large room.
Floor 3a
Ho, hum... just another inane floor checkered with lava patches. There's no treasure here, just head to the right and you eventually see the stairs to floor 4. The path to the left of the stairs leading back to floor 2 is a dead end, so don't bother.
Floor 4a
There are actually two separate sections of floor 4 (and floor 3, for that matter), so you'll have to go down, up, and back down to get to floor 5. You'll appear in a wide lava field with occasional "islands" of safe ground to step on There are no chests here so just hotfoot it to each island and you should see some stairs at the bottom right side of the floor that head back up.
Floor 3b
This floor is a lot like the floor 3a, except that the entire floor is broken into a series of small rooms and some long corridors with wide lava pools. There's nothing here that you haven't dealt with before, so walk (or run) left past two of the rooms, then go all the way down, then go all the way to the left again. Stairs that go back down? Yes!
Floor 4b
Unlike most the other floors here, this one has lots of treasure, including the vaunted Ice Brand. It's the strongest sword you'll have for quite awhile. Too bad for the stupid sound it makes (it almost sounds like it's going to break every time you use it). The stairs to floor 5 are at the very bottom. It shouldn't be very hard to find.
Floor 5
Well, you're finally near the fiend of fire. There's a choice of direction here. Actually eight directions but the two of interest are the southwest path and the west path. Head down the west path first for a nice piece of equipment. Once you've collected the contents of the chest, walk down the other path to find the orb. Touch it.
Marilith
Marilith has strong physical attacks that can do more than 100 damage to a Warrior and well over 200 to a White Mage or Black Mage. In addition to that, she has several mid-range spells that can wear you down easily. The first thing you should do in this battle is cast Silence to stop her spells, or just NulFire and NulBolt to lessen the amount of damage caused by her spells. After that, your White Mages should cast Blink, Shld2, Invs1/Invs2 until Marilith can't hit you anymore.
As for your Black Mages, they are out of luck. Marilith has a good resistance to all of his spells, even ice, which you would think she would be weak against. Just use up your strong offensive spells like Ice2, Bolt3 (if you have it), and Bind, which will work well if she continues to pound on your mages. Curiously, Fire3 actually does some damage to Marilith. Your physical attackers will be doing much of the damage here so just use your Mages be backup. After all of that, she should bite the dust soon.
Touch the crystal again and then enter the warp to get sent to the surface.