In Venezzia, you learn Demitel's whereabouts and can proceed to his island.
Explore Demitel's Island
After you arrive at the island, you can rest on the ship for free by going through the door between the two stairs, then go to the southwest corner of the room and go downstairs, then go to the southwest corner of this room and go into the door there. You find beds and a save point.
Before you leave the dock, you can find two chests there. The one on the left contains a Mixed Gummy/Melange Gel. The one on the right contains a ?Book, but it turns into a weak weapon, so you might as well leave it in the chest.
Go north from the dock to reach the area outside of the mansion.
Go north and up the ramp, then go west and open the chest to get a Spy Lens/Magic Lens. Go north and there will be a cutscene as you approach the mansion.
Demitel's Mansion
Go into the mansion, and go west. The first door has a save point. Go south from there and you will see two doors. The one on the left is locked. Go through the unlocked door to find Demitel's library.
In Demitel's library, go to the upper-left corner of the room and open the chest that is hidden behind a vase. It contains Savory. Then check the shelf behind that to get a Magic Key.
Go back to the first room of the mansion and go east. Go through the door on the right to find several chests. From top to bottom, left to right, they contain an Apple Gummy/Apple Gel/Apple Gummi, Verbena, Savory, Apple Gummy/Apple Gel/Apple Gummi, Elixir, and 430 gold.
Then exit the treasure room and use the magic key on the upper door.
In the garden, there are random enemy encounters.
Walk up to the big tree in the center of the garden. You will soon fight a mini-boss, so you should prepare now. Equip Cless/Cress's Lightning Bolt skill. Change Klarth/Claus's tactic to summon the Sylphs. Leave Mint's tactic to Value HP over TP. Check the tree until the battle starts.
Oak Roots
Use Lightning Bolt against this enemy. If you have trouble, you can manually make Mint cast Hammer.
Demitel's Island, Part 2
When the tree is gone, change Klarth/Claus's tactic to "Don't Call Spirits". Go back into the first room of the mansion. Open the middle curtain, then check the middle prism twice, and check the left and right prisms three times. A new door will open. Before you go through, go back to the ship to heal up.
Then go back and use the passage that you just opened. At the first fork, go west to find a chest that contains a Reverse Doll, then go east. At the next fork, go south to find more chests. The one on the left contains a Corsesque and the one on the right contains an Armet Helm. These are good to sell if you already have them. Then go back to the fork in the path, and go north this time. Before you go through the door, prepare for the battle with Demitel.
Be sure to equip Cless/Cress with a Reverse Doll or two, equip Mint with a Mystical Rune and a Talisman, and equip Klarth/Claus with his Opal Ring. Have Klarth/Claus's tactic set to "Don't Call Spirits", because during the battle you will want to use the menu to manually have Klarth attack Demitel with the Sylphs. Have Mint's tactic set to value HP over TP. Cless/Cress should equip Lightning Bolt for his long range skills.
Demitel
All of the enemies' spells are very dangerous, so you need to try to interrupt enemy spells by focusing your attacks on any enemies that are casting spells. It is especially important to interrupt Demitel before he can cast Summon Lich, which may defeat you instantly. Use the menu to have Klarth manually cast Sylph and have Mint manually cast Hammer to try to interrupt enemy spells. Once you have killed the enemies between Cless/Cress and Demitel, try to have Cless/Cress keep his distance and constantly use long-range attacks and skills on Demitel. If Cless/Cress is too close, Demitel will use his close-range attack on him, which does high damage. Use Life Bottles on anyone who has died. If Mint is dead, use a Life Bottle on her before anyone else. Use Gummies to restore health and TP whenever possible as well.
Afterward
After you defeat Demitel, there will be a cutscene, and you will meet Arche.