Before going to Moria, you seek a contract with Ifrit.
From the entrance, go east and north to find a chest that contains a Spy Lens/Magic Lens, then go south and down the stairs.
Go south from the stairs to find chests. The one on the left contains a Talisman, and the one on the right contains a Resist Ring. From there, go west and north to get the Sorcerer's Ring. You should equip it on someone in your party now. You can use it to shoot sparks. Now go east and go up the stairs.
Go west to find a locked door. Walk on the column in the lava until you are facing the switch in the wall. Make sure someone has the Sorcerer's Ring equipped, then press the Accept button to hit the switch and open the door. Go through.
Go north to find chests. From right to left, they contain a Savory, a ?Weapon that becomes Thunderclap, and a Battle Rune. Equip the Battle Rune on Cless/Cress now. From there, go south, and go down the stairs.
Follow the path until the path splits. Go north up the stairs to find chests. The bottom-left one contains a Mixed Gummy/Melange Gel, the upper-left one contains 1000 gold, and the one on the right contains the Magma Key. Then go south, then east, then south. You will be back in the room where you got the Sorcerer's Ring. Go east until you see the stairs, and go up the stairs.
Now go west and through the door that you see. From there, go west, open the chest to get a Charm Bottle, and use the Sorcerer's Ring on the switch on the back wall. Go back to the save point and go north through the door.
In the next room, check on the locked door to use the Magma Key. Go north and you will see some columns. Some of the columns are standing up. Use the Sorcerer's Ring on the columns that are standing up, and they will fall, giving you a path through the magma. Go through the room this way to reach Ifrit.
Cless/Cress should not use Phoenix. Otherwise, he should use his best skills.
Ifrit's Cave, Part 2
Open the chest behind where Ifrit was. It contains Fire Wall, a spell for Arche. Disable that spell for now. Go into Klarth/Claus's skill menu and disable Ifrit.
Go south to leave Ifrit's Cave, then re-enable Arche's fire spells and Klarth/Claus's Ifrit.
When you are out of the cave, you can change Cless/Cress's weapon, if you equipped the Ice Scimitar before. Change Klarth/Claus and Arche's tactics to not cast spells, but re-enable their fire spells.
Your next destination is Undine's Cave. To get there, go west from Ifrit's Cave to reach the dock. Go into the dock and take the ship back to Alvanista. Go north and then west to Alvanista.
Before you leave Alvanista, if you can afford it, you can buy Rune Bottles to turn your three Flame Mantles into Aqua Mantles. If you don't have Flame Mantles, you can go to Beladum to buy three Mantles, then use Rune Bottles on them to turn them into Flame Mantles, then use Rune Bottles on those to turn them into Aqua Mantles. Equip Arche with the Mental Ring. Equip Klarth/Claus with the Aquamarine Ring.
If you are still in Alvanista, go to the Alvanista Harbor and take the ship to Venezzia. Then, in Venezzia, talk to the captain of the ship on the left and ask to go to the North Island.