After you receive the Unicorn's help, you go to Venezzia to attempt to reach the Abyss of Thor.
Explore the Abyss of Thor
When you arrive in Thor, go into the building to the north. Check the sparkling item on the counter to get the Magnetic Card. Then leave the building.
Go south, down the stairs, then east. You will see a save point. Go north from the save point and check the panel to the left of the door to use the card. Go through the door.
Use Lightning on the enemies in this area.
Follow the path, and you will reach a big room. Go directly south. You may be able to avoid the monster. In the south part of the room you will find some chests. One of the chests holds a Skeleton Key that you can use to open one door. Three of the doors will open with the key, but they are random. Note that the Skeleton Key disappears after you unlock a door, so if you want to try other doors after you unlock one, you will have to come back to this room and open the chests in the south part of the room until you find it again. One of the rooms contains the God Breath spell for Arche. You might want to turn off that spell after she learns it since lightning spells work best on the enemies here. If you find yourself in the hallway that brought you to this room, you chose the wrong door and will have to start again.
Another of the rooms that you can find is a room with a save point. In the save point room, equip Arche with a Mystical Rune, then equip Mint with one if you have another. Equip Klarth/Claus with the Moonstone ring, and equip Mithril Meshes on two of the others. Equip Cless/Cress with the Lightning Bolt skill if you haven't already. Set Arche to not cast spells.
Change your formation to put everyone slightly to the left of the middle. Go north and there will be a battle.
2SHs and 4400Rs
Don't stand too close to the enemies because they will shoot lasers at you if you are underneath. Use Indignation on the floating 4400Rs first, then on the two 2SHs that are on the ground. Have Klarth/Claus cast Maxwell on the 2SHs right away. Cless/Cress should use Lightning Bolt.
Go north to use the computer. First use the Medical Treatment. Then try Time Warp. Say Yes to restore power. Then go back to the save point and save the game, then get ready, because you're about to face Dhaos.
Prepare for Battle
Equip the Diamond Ring on Klarth/Claus. Turn off Klarth/Claus and Arche's spells so you can cast them manually. As before, equip Arche with a Mystical Rune, and if you have another, equip it on Mint. Equip Cless/Cress with his most powerful skills. Tempest and Blade Storm work well to stun Dhaos. Change your formation to put Cless/Cress on the rightmost side, and everyone else on the leftmost side.
Go back to the computer and choose Time Warp.
Battle the Enemy
Hit Dhaos with Tempest or Blade Storm constantly, and have Klarth/Claus cast Maxwell and Arche cast Indignation as fast as you can. Use a Flare Bottle on Cless/Cress and have Mint cast Haste on him. Mint can also cast Hammer on Dhaos. Try to have Cless/Cress keep Dhaos far away from the others, because Dhaos's physical attacks can petrify those it hits.
After the cutscene, you will be in the town of Miguel.