Klarth/Claus must forge a contract with Undine to get her assistance in the fight against Dhaos.
Go to the Glaciated Cavern
To go to Undine's cave, first go to Venezzia. If you have been following Nancy and Elwyn's story, talk to Elwyn's father upstairs in the trading company, and tell Doug where Elwyn went. Then talk to the captain of the ship on the left side of the Venezzia harbor. Ask to go to the North Island. (If this is not an option, you have to go back to Alvanista and talk to the two elves in the room across from the prince's room.)
Glaciated Cavern
When you fill your food sack in Undine's cave, use Seafood to fill it, because you will often win Seafood from monsters in the cave.
Equip your party with with Aqua Mantles if you have them (use a Rune Bottle on a Mantle to turn it into a Flame Mantle, then use a Rune Bottle on the Flame Mantle to turn it into an Aqua Mantle). Equip Arche with the Mental Ring. Equip Klarth/Claus with the Aquamarine ring.
When you are ready, go east from the entrance and follow the path to reach a bridge. Cross the bridge and pull the lever. Then go north past the bridge, then go west and south to the next room.
Go south and get the treasure. The upper chest contains an Orange Gummy/Orange Gel/Orange Gummi, and the lower chest contains a Spy Lens/Magic Lens. Then go down the southwest stairs.
In this room, open the chest to get a Rune Bottle, then pull the lever, then go back up the stairs.
Go north and go up the stairs on the left, and go through the door there. Then go east, south, and west to the switch that you pulled earlier, and pull it again.
Go north past the bridge, and this time go through the door to the north. Follow the path, open the chest to get an Aqua Mantle, and go north and through the door.
In the next room, go north, past where you see the chest, then go west to find some stairs leading down. Go down them, open the chest to get Seafood, then go west and then south and down the stairs. Throw the switch and go back up.
Go north to the next room. Follow the path, past the save point, until you reach two levers. Pull the one that is farther to the north.
Go back the way you came, and you will see that the other pool is empty now. Go into that pool, open the chest to get Seafood, and go down the stairs. Pull the lever.
Undine is in the room with the save point and the two switches. You might want to go back to the ship to rest first. This will help you level up before the battle. You can also buy supplies from the guy in the northwest corner of the big room on the ship.
Before you fight Undine, equip Mint with a Mystical Rune so she can heal your party quickly, and leave her tactic at Value HP over TP. Equip Cless/Cress with a couple of Reverse Dolls. Set Arche and Klarth/Claus to cast many spells. Turn off Arche's Tractor Beam, Storm, and ice and water spells. Cless/Cress's Tiger Teeth and Lightning Bolt are recommended for this battle. For your formation, put Cless/Cress in the middle of the screen and put everyone else on the leftmost side.
If you are ready to fight Undine, go north to the room with the save point, save the game, and pull the southernmost switch.
Undine
Keep Cless/Cress far away from Undine and have him use his long range attack. Use Gummies to supplement Mint's healing.
Undine's Cave, part 2
When the battle is over, use a Rune Bottle on the ?Book that you won. It becomes another Rabionis, which you can sell. Then set Klarth/Claus and Arche's tactics to not cast any spells. Also, change your formation so that everyone is in the middle again.
To exit the cave, pull the lever on the left to drain the pool, then go east and south from the save point to the next room. Go south through this room, then follow the straightforward path until it splits. At the split, go east, then south past the bridge, and west to that lever that you pulled earlier. Pull it now. Then go north past the bridge, then go west, then south to the next room. Then go east and up the stairs on the right, then go east, then southwest to leave the cave. Talk to the ship captain to return to Venezzia.
Venezzia
In Venezzia, tell Elwyn's father where his son went if you haven't already.
Alvanista
Elwyn, Elwyn's father, and Nancy will all be in the northeast house in Alvanista, in the middle of an argument. Help them. In the SNES version, that completes the side quest for Elwyn and Nancy. In other versions, you can go back to the trading company in Venezia and talk to Elwin to attend to Nancy and Elwin's wedding. You will get a reward for Mint if you do so.
Moving On
Buy two Rune Bottles in Alvanista if you can afford them. Stock up on Remedy Bottles as well. Equip Cless/Cress with Gungnir or another non-elemental weapon.
Make sure you have your Moria Pass in the Treasures section of your inventory. If you don't have it, talk to the guy behind the counter in the Adventurer's Guild in Alvanista. The Adventurer's Guild is through the passage to the left of the counter in the Alvanista Pharmacy.
Leave Alvanista to go to the overworld, then go east across the bridge, then go south. When you are past the mountains, go east, then go northeast to find the entrance to the Moria Gallery.