Cless/Cress and his friends can safely pass through the Valhalla Plains and reach Dhaos's Castle.
Enter Dhaos's Castle
After you have gone northeast through the Valhalla Plains, you will be in the overworld. Save the game, then cross the bridge and go north to reach the castle. Cless/Cress should use Lightning against the enemies here. Make sure that Cless/Cress has a non-elemental weapon equipped, such as the Mecha-Halberd (press Y to display the weapon's element, if any).
Iron Man: use Indignation and Luna. Absorbs fire and earth.
Vine: weak to Fire. Use Ifrit, and Fire Storm (learned in Dhaos's Castle).
Draygen: weak to Fire. Use Ifrit, and Fire Storm.
Druid: use Fire Storm & Maxwell.
Dullahan: use Ray and Luna.
Charon: use Maxwell. Absorbs fire.
Adept: use Ray and Luna.
Draygens occasionally drop ?Weapons, and sometimes the ?Weapon is a Moon Falux. Otherwise, it is just a Longsword. You will want the Moon Falux, but don't equip it now.
Charons can cast Heal, and Druids can cast Cure and Barrier, so you might want to attack them with spells before you attack the other enemies.
Get Fire Storm
In the first room of the castle, go west. Go into the leftmost door, open the chest to get a Flame Mantle, and go east to find another chest, which contains an Aqua Mantle. From there, go south.
Go east and check behind the statue to get a Mystical Rune. Go south from there, then go southeast and down the stairs, then go slightly east until you can go north. Go through the door there.
Go north to reach three weapon stands. Check on all three. From right to left, they contain a ?Weapon that becomes a Slayer Sword, a Mecha-Halberd, and a Halberd. Exit this room, then go east.
In this room, check the plant to find a Savory and Sage, then go east. Go north and step on the switch, then go north through the door that just opened. Open the chest to get a new spell for Arche: Fire Storm. Then exit the room.
Get the Magic Mirror
Go to the north wall and check on the two weapon stands. From right to left, they contain a Halberd and a Mecha-Halberd. Then exit south. Go east again, and step on the switch. This will unlock the leftmost door. Go through it.
Follow the path. When you reach the two doors, go through the one on the right. The correct order to step on the buttons is: up, down, right, left, down, up. If you make a mistake, just exit and go back in, or check the sign. After you do it correctly, exit the room and go through the door on the left. Then go north through the open door.
Go east and you will notice your reflection in the mirror. Go east and go up the stairs to the next room.
At the crossroads, go north, then go east and go up the stairs. At the three doors, go through the left door. There is an Orange Gummy/Orange Gel/Orange Gummi on the left and a Life Bottle on the right. Exit the room, then go through the door on the right. The bottom left chest contains an Apple Gummy/Apple Gel/Apple Gummi, the bottom-right chest contains a Rune Rod, and the chest in the north part of the room contains an Apple Gummy/Apple Gel/Apple Gummi. Go south to exit this room, then go through the middle door.
Go west first to find some chests. From left to right, they contain an Orange Gummy/Orange Gel/Orange Gummi, a Life Bottle, and an Apple Gummy/Apple Gel/Apple Gummi. Then exit the room and go east, and go north and through the door. You will reach a room with switches on the floor. Go north and read the sign first. Then you will have to step on the switches. Try to step on each switch right after Arche does.
After you do it successfully, go through the door that opened. Use the save point, then go up the stairs.
You reach a room with a door in the north wall, and another door across from that door. First, go through the south door to find some chests. The one on the left contains a Rune Bottle, and the one on the right contains a Protect Ring. Then exit the room, and go through the north door.
Go through the leftmost door in the north wall to find a chest that contains a Battle Rune. Exit the room and go through the southwest door to find a chest that contains a Steak. Then exit the room and go through the rightmost door in the north wall.
Go up the stairs, then go south to find a Demitel statue. Push the Demitel statue north into the hole, then go back downstairs and push the Demitel statue onto the button. Go through the door.
Step on this switch twice and choose someone to stay behind. It is recommended to leave Arche behind. If you want someone else to stand on the switch, talk to whichever party member is standing on it currently. Go through the door.
Follow the path. When you see a locked door to the east, go west. Keep going west until you reach some stairs. Go north up the stairs.
Go west and follow the path until you reach two doors. Go through the rightmost door to find a save point. There are random encounters in the save point room, so just save and then go south to the previous room. Go through the door on the left, then go through north door, and prepare for a mini-boss: an Evil Lord and some Draygens. If you haven't used Rune Bottles on the ?Weapons in your inventory, use one on each separate stack of ?Weapons. If one of them turns into a Moon Falux, equip Cless/Cress with it now.
Klarth/Claus should use Luna, or, if you have Arche, she should use Indignation. Mint can cast Hammer on the Evil Lord throughout the battle.
Re-equip Cless/Cress with a non-elemental weapon, open the chest that was behind the monster to get the Magic Mirror.
Get the Gold Key
Exit the room, use the stairs to the southwest, go down the stairs, then keep going to the east until you reach two doors. Go through the one on the left, and keep going until you reach the person you left behind. Talk to that person and choose to leave nobody behind.
Go south, then keep going directly south until you are in a room with a tapestry on the right. Go south from that tapestry, down some stairs. You will pass a save point. Keep going south until you are in the room with the diamond-shaped column. There are mirrors to the left and right. When you check the mirrors, you will fight an Evil Lord and Draygens again, so equip Cless/Cress with the Moon Falux first, and prepare Arche to cast Indignation and Klarth/Claus to cast Luna. Check each mirror and fight each monster, until you are taken to a new room. In the room on the left, the chests contain (from left to right) Lavender, 10000 Gold, Hourglass, Elixir, and White Mist. In the room on the right, the chest contains the spell Distortion. Disable this spell for now. After you get the treasure, exit by checking the mirror.
Now go south down the stairs, re-equip Cless/Cresswith the Moon Falux as before, and check the nearest mirror to fight another mini-boss. After you are transported, re-equip Cless/Cresswith a non-elemental weapon. Go west and check both gargoyles and say Yes to make a chest appear (Mental Ring), then open the other chests. From left to right, they contain a Magical Broom, Rune Bottle, and Armlet. Equip the Magical Broom on Arche now.
Go east past the mirror, then go south. Follow the path and you will reach a room with four switches. Press them in this order: up, down, left, right, down, up. After you do it correctly, exit the room to find a chest that contains a Gold Key.
Go back the way you came, then check on the mirror. Go east and up the stairs. Go north past the column until you can go east and up the stairs there. Go through the middle door, then go through the upper-right door, then go north in the room with the table. Save the game if you wish, then go north up the stairs to the next room. Go through the upper door, go through the northernmost door, and go north into the room with the switch that someone has to stand on.
Step on the switch and leave Arche behind. Go through the door, go up the stairs to the next room, and go south through the door. Now check the door on the right to unlock it, and go through.
Walk to the right, and Arche will come join you. Open the chest to get a Charm Bottle, and go back, out to the hall, and go west to the next room. Use the south exit. Unlock the door to the left and go through to find some items. The bottom-left chest contains a Dragon Steak, the bottom-right chest contains a Silver Cape, and the northern chests both contain Dragon Steaks.
Exit and go through the door on the right. Go west until you reach some stairs. Go up those stairs to reach the next room. Then go east and south, then through the door on the right. Save the game if you wish, then check the door in this room, and the four of you will unlock it.
Go through, and prepare for another mini-boss fight with an Evil Lord and Draygens. As before, equip Cless/Cress with a Moon Falux if you have one. Klarth/Claus should use Luna and Arche should use Indignation. Mint can cast Hammer on the Evil Lord throughout the battle.
After the battle, you might want to go back to the save point and save the game, because Dhaos is next. You will have to check on the door north of the save point to unlock it again.
Go north from the save point room to reach Dhaos's room. You will not be attacked in Dhaos's room, so you can walk around to heal up with the food in your sack.
Replenish TP. Use Mixed Gummies if you have to.
Change Klarth/Claus and Arche's tactics to not cast spells. It will be better to cast their spells manually. Set Mint to Value HP over TP. Make sure that Arche is equipped with a Mystical Rune, and if you have more Mystical Runes, equip one on Mint, and if you have one left, one on Klarth/Claus. Set Cless/Cress to use Tempest and Blade Storm. (Tempest can be purchased in the Venezzia Armor Shop if you did not get it before. It requires mastering Lightning Bolt and Blade Storm.) Blade Storm won't hurt Dhaos, most likely, but it does have a good chance of stunning him, which will keep him from casting any spells for a short time. Set your formation with everybody all the way on the left of the screen, with Cless/Cress on the rightmost side.
Dhaos will not attack very often if the Evil Lords are still alive. However, you will have to occasionally attack the Evil Lords to prevent them from killing you.
Arche can cast Lightning or Ice Needle on Dhaos to interrupt his spells, but Arche should mostly cast Indignation, which deals massive damage to Dhaos.
Klarth/Claus should constantly attack Dhaos with Luna.
Have Cless/Cress constantly attack Dhaos with Blade Storm in the hope of stunning him.
After the Evil Lords are both gone, Mint should cast Hammer on Dhaos repeatedly.
Afterward, there will be a cutscene. During the cutscene, say Yes to agree to save the Tree of Life.
During the cutscene, you will be automatically healed up. After the cutscene, you should go to Midgard. You will need to go east to Alvanista first. If you didn't race the kid in Alvanista to get the Cat's Eyes, do so now. You won't be able to later.
From Alvanista, go east and south to the dock to Freland, take the ship to Freland, then go east past Olive Village and cave entrance, east through the mountain pass, and go northwest from there to reach the bridge. Cross it and go north to Midgard.
Use a Rune Bottle on the Jade Ring that you won from Dhaos to turn it into a Fairy Ring.
Buy supplies in Midgard if you need them. Rest in the inn and watch the scene if you didn't see it before.
IMPORTANT: Before you leave Midgard, talk to the young woman in the north part of the city who is standing by a tree.
Then leave Midgard and go northeast to the Valhalla Plains.
From the start of the Valhalla Plains, to reach the White Forest, go north three times, east, north, east twice, north four times, west, north, west twice, and north. You will be in the overworld. Go west into the White Forest.