After you learn that sea travel is unsafe now, you seek a contract with Volt to power up Stanley's invention.
Explore Volt's Cavern
If you fight Bone Bows here, stay as far away as possible. Use Indignation on them. They will sometimes drop ?Bows. If you use a Rune Bottle on it, it might turn into a Shortbow or a Wave Spread. If you get a Wave Spread, equip it on Chester.
Volt Flies absorb lightning, fire, and light, so don't use Indignation, Fire Storm, or Ray on them. God Breath and other wind spells will work.
Go north to find a Holy Bottle, then go through the nearby door to find chests that contain Lavender. Go back inside, and go west, and open the chests. The north chest contains a Life Bottle and the south chest contains a Shield Ring. Use the Shield Ring on anyone but Cless/Cress. The Shield Ring is a shield, NOT an accessory.
Go east twice, then north. Equip the Sorcerer's Ring and shoot the coil to open the door. Go through and open the chest to get a Grimorum, a weapon for Klarth/Claus. Then go east and follow the path.
Walk slightly south, then go directly east to go through the wall. Step on the golden switch, then go north through the open door. From there, ignore the door to the north, and instead go east.
Outside, if you do NOT need an Orange Gummy/Orange Gel/Orange Gummi, go south to the next area, and skip ahead to the next paragraph. Otherwise, go east and go north through the door. Keep going, and open the chest to get an Orange Gummy/Orange Gel/Orange Gummi, but don't go through that door! Go back outside, and exit to the south.
Go southeast and into the room, hit the switch, then shoot the middle coil to open the northeast door. Run through, then go east and down the stairs.
Open the chests. The one on the left contains an Apple, and the one on the right contains Bread. Then go through the door. Make sure that you have Jet Boots equipped. Notice that to the right of the coils, there is a dent in the wall. Shoot the coils with the Sorcerer's Ring, then run east through that dent in the wall, then go north through the door. After that, go through the door on the right.
Go through the door above and hit the switch, then go back through the door and go east, then north, to find a chest that contains a Sylph Broom. Equip it on Arche. Then go south to the next area.
Go south, then west through the thin wall, then go all the way to the west side of the area. Go north to find another group of coils. First, hit the switch on the back wall, then use a Holy Bottle if you have one, shoot the coils, and run through the door that opened. Then go through the first door in the north wall.
Save the game, then go east and stand between the easternmost coils and the middle set of coils. Shoot the eastern set of coils, then quickly shoot the middle coils before the eastern coils deactivate, then run to the left and shoot the westernmost set of coils before the coils deactivate, then run through the door.
When outside, make your party consist of Cless/Cress, Arche, Klarth/Claus, and Mint. Equip Klarth/Claus with a Mystical Rune and Holy Cloak. Disable all of Klarth/Claus's spells except Maxwell. Disable Arche's Indignation and Ray, and get her ready to cast God Breath if she has it. If Cless/Cress has the Moon Falux equipped, switch to a non-elemental weapon.
Go north to fight Volt.
Volt
Stay away from the Arurouns, and have Cless/Cress attack and the others use the spells mentioned above.
After the battle, go back the way you came, and there will be a cutscene. Use a Rune Bottle on the ?Armor to turn it into a Star Cloak. Equip it on anyone but Cless/Cress or Chester.
Go south to the room with the three sets of coils, save the game if you wish, then go east and step on the teleporter to be returned to the entrance. Go south to go back outside. Rest at Morrison's house, then go west and then north to the mountain pass. Go through the mountain pass, then go north to Euclid, enter the northwest house, and talk to Stanley downstars. Afterward, you can travel the world.