Take the boat back to Venezzia, and take the other boat (the one you used to get to Demitel's Island (West Island) to get to North Island.

There's a map I made of this area (here) ifya wanna look at it while you read this. Inside the cave go left, up and around, and down the stairs. The door locks behind you. Cross the bridge and hit the lever at the bottom-left, then go down the stairs at the bottom left to get an Orange Gummy and a Spy Lens. Now that you're back in the main cavern, I suggest you go back to the ship to heal. When you get back, go down the stairs at the bottom-left of the screen and go down and around to get the Rune Bottle. Flip the switch, then go back to the main cavern, up through the door, and around to hit the first switch again. Enter the top door now and grab an Aqua Mantle out of the chest. Go up the stairs.

Left and up, then down the stairs is Seafood in a chest. Go down the stairs and flip the switch. Go straight to the back, go down into the hole, up the stairs, flip the switch in the back, and go back into the last room to get the Seafood out of the previously-unattainable chest. Go back into the back room, save and heal at the SP, then flip the other switch (the one directly in front of the stairs). The room fills with water, and the guardian of the cavern appears...


Boss: Undine, Calamari (2), Red Slug (2)
HP: Undine: 6,450, Calamari: 2,243, Red Slug: 650
TP: Undine: 300, Calamari: 0, Red Slug: 0
Attacks: Delay, Ice Wall, physicals (all-party)
Recommended level: 23
My current level: Cless 22, Mint 21, Klarth 21, Arche 20

Alright, have all of Arche's fire and lightning spells on automatic, and leave Sylphs turned off in Klarth's menu. Equip Cless with the Aqua Mantle and Klarth with the Aquamarine Ring. This way, neither of them get hurt by Undine's Ice Wall. You probably should have the Black Onyx equipped on Cless (that thing that increases your HP by, like, 30%? You got it from the fight with Jahmir). As usual, have Mint cast Delay early on, once on the enemies and as many times as you have to on Undine until it doesn't Fail. Have Cless use Phoenix or Burning Soul, if you have it (I prefer Phoenix, but whatever floats yer boat, dude) If you don't have those, then use whatever LR tech you've got, but fire works best. Have Mint cast Nurse after every physical attack by Undine (when she sends that light blue thing across the screen). She should cast it on herself so that it heals everyone else while Cless is off-screen hitting Undine with Phoenix. If she doesn't have Nurse yet, have her cast Heal on Klarth, Arche, and herself. Cless should be fine. *Note: It is possible to beat her with Cless at lv. 19, Mint 18, Klarth 18, and Arche 16, but jeebus, that was insanity. ^.__.^

After the battle, Undine apologizes and contracts with Klarth. Flip the switch in front of you to drain the water again, then leave the Glaciated Cavern. Go south, up the stairs, south from the SP, south through the next room, down the stairs, up the stairs to the right, south some more, right and down, flip the switch, right up and left, down the stairs, up the stairs to the right, south again, and out the door. Talk to the captain to head back to the mainland.