Ok, just a quick note. This is probably THE longest dungeon in the game, far worse than even the final place you have to go to. It's 10 floors long and it'll really tax you along the way, IF you haven't been training. Therefore, I strongly suggest you make sure you have at least 1,500 HP for this place, decent Equipment and spells, as well as weapons/Shield Levels. MP wise, at least 250-300 is good and I PRAY you have Osmose, which you got back at the Mysidian Cave. This WILL be your saving grace when you run out of MP. Do NOT train it, as you really and truly don't need to, but Level 3-4 is generally good. Don' bother buying Ethers, as they really don't give you much MP back, so just rely on Osmose.
Anyway, once you're ready, head inside and as you start out, head northwest to find a Treasure Chest which holds a Staff, then head northeast to see a Treasure Chest inside a wall and try to enter the wall to the east and open the Treasure Chest here to get a White Robe. Give it to whoever you wish, as it's slightly better than what a few of your guys have, then head east and grab the Treasure Chest here to get a Staff. After this, head southeast and grab the next Treasure Chest you see to get yet another Staff. Wow, useful stuff, huh? Finally, just head west and go up the stairs here.
On this floor, head northeast and follow the path around to find 2 Treasure Chests that hold a Flame Bow and a Flame Lance. Both are probably outdated to you by now, so, heh, don't use them. Next up, head southwest as far as you can go, then head north to find a Treasure Chest, which holds a Flame Shield, also outdated to you by now as well. Afterwards, head south as far as you can go, then head east to find the stairs, so, head up them now. On this next floor, head west and enter only the door farthest to the west and take note of the lava that hurts you for 1 HP per step here as well. Then, in the next area, head east as far as you can to find a Treasure Chest, which holds a Flame Sword. Give this to Firion if he's your Swordsman or whoever else and make sure to give Gareth the other Sword the person was just using now.
Next, head back west and get onto solid ground now and then head north and then head east and west to find 2 Treasure Chests, which hold a Flame Helmet and a Flame Mail. Equip the Flame Helmet on whoever you wish that needs it the most, but chances are you won't need the Flame Mail. Either way, afterwards, enter the door nearby and gear up for a Boss Battle. If you want a strategy for this, head to Section VII. Boss Guide now, otherwise, win this yourself and then move on. After the Battle, head up the stairs to the north and on this floor, head west and open the Treasure Chest here to get an Ice Shield, which is also outdated by now possibly and then head south and up the stairs here. On this floor, head south and watch out for the ice spikes here that damage you for 1 HP per step and grab the Treasure Chest here to get an Ice Lance. If anyone uses Spears, give this to them, as it's better than the Flame Lance.
After this, head back down the stairs and on this floor again, head south and grab the Treasure Chest here to get an Ice Gloves. Give them to whoever you think needs them the most, then continue east and head northwest when you can to find a Treasure Chest that holds an Ice Mail. Again, give it to whoever you wish and afterwards, head northeast and go up the stairs. On the next floor, just follow the path over the ice spikes and grab the Treasure Chest you come across to get an Ice Brand Sword. Don't Equip it just yet, as the next Boss is Ice based, so you'll want the Flame Sword on whoever your Swordsman is. Afterwards, do Equip it on whoever is the Swordsman and give Gareth the old weapon they were using. If not, just give Gareth the Ice Brand Sword.
So, head through the door here and get ready for another Boss Battle. If you'd like a strategy, head to Section VII. Boss Guide now, otherwise, fight this yourself and then move on. After the Battle, head up the stairs here and on this floor, head east and grab the Treasure Chest here to get a Potion. Then, head back west and go south now and then head east as far as you can and then head south and follow the path until you reach a Treasure Chest that holds a Spellbinder Staff, which, if anyone uses Staves, give this to them now. Afterwards, head through the door in the southwest and in this room, head up the stairs to the north. On this floor, head east to find a Treasure Chest that holds a Silk Web, then head northeast and grab the Treasure Chest here to get a Hermes.
Next up, head west and open the Treasure Chest you see to get a Unicorn Horn, then continue west and enter the door here, as you enter another Boss Battle. If you want a strategy for this, head to Section VII. Boss Guide now, otherwise, fight this yourself and then move on. After the Battle, head up the stairs here and on the next floor, head north as far as you can and then go east to find a Treasure Chest, which holds a Flare Scroll, guarded by Gottoses and Evil Buds. Afterwards, DEFINITELY use Flare on whoever you wish, as it's a GREAT Magic spell and after this, head south and then go east when you can and enter the door here and in this room, head up the stairs to the north.
On this floor, head north and grab the Treasure Chest here to get a Hourglass, then head west and get the Treasure Chest you see to get a Cross. Next up, head north as far as you can, then go east and open the Treasure Chest here to get a Black Robe, which is guarded by a White Dragon. This thing is VERY strong, so fight at your own risk. Afterwards, give the Black Robe to whoever needs it, then head southeast to find 4 doors and enter the one that's the second on the left and in this room, head up the stairs to the north. On this floor, just head north to find Mindu. He tells you the situation and that he needs to throw everything he has at the seal, as he orders them to stand back. He manages to destroy the seal, but he kills himself in the process.
After his death scene, enter the door to the north and in this room, examine the four orbs here and NOT the middle one just yet, as these four orbs will boost a certain stat for a random party member. Strength, Spirit, Intelligence and Agility is what will be increased. Finally, examine the orb in the middle of the room to get the Ultima Scroll and after this, head to the teleporter that appears and you'll get out of this place.