With the Airbirds, you can reach Odin's Tower in the mountains near Olive Village.
Train Cless and Get Nymph Rings
Odin's Tower is in the mountains of the Freland continent, surrounded by lava, near Ifrit's cave. It is south and slightly east of Freezekill.
Your party should consist of Cless/Cress, Mint, Klarth/Claus, and Arche.
In the tower, equip Cless/Cress and Mint with Flame Mantles. Put the Garnet Ring on Klarth/Claus. Make sure that Cless/Cress isn't set to use any fire-based skills. For Arche, disable everything except Tidal Wave. For Klarth/Claus, disable Ifrit, Sylphs, and Gnome. Use a Fairy Ring on Arche.
If you haven't trained Cless/Cress with Soul Edge yet, you can tell Arche and Klarth/Claus not to cast spells, and walk around the entrance and use Soul Edge on all of the monsters. If you do this, though, you will have to use Orange Gummys/Orange Gels/Orange Gummis or leave the tower and go to Olive Village to rest (or go northeast to the Long Valley to rest for free), because Soul Edge consumes a lot of TP.
There is another good reason to fight enemies in the entrance of the tower: you can get fire-protecting rings from some of the enemies. If you fight Karys, they sometimes drop Salamander's Rings, which become Nymph's Rings with a Rune Bottle. Nymph's Rings make you immune to fire spells, and immune to the damaging effects of the lava.
You can stop after getting four Nymph's Rings, but be aware that your fifth, inactive party member will die in lava rooms if not equipped with a fifth Nymph's Ring.
When you're ready, you can explore the rest of the tower.
Explore Odin's Tower
From the entrance, go west, and go north up the hallway and flip the switch. Then go south down the hallway, and go through the door on the left. The left chest contains a Basilisk Scale, and the chest on the right contains an ?Armor that becomes a Reflect Plate. Go back out.
The door south of the room that you just came out of leads to a dead end, but you will not encounter enemies there, so you can use the room to heal up. It is the same room that you went to when you met the Valkyrie in the past.
From the first room of the dungeon, go east, past the entrance, and go north up the other hallway and flip the switch. Then go south down that hallway and go through the southeast door. Open the chests. The one on the left contains a ?Book that becomes Seventh Sun, and the one on the right contains a Moon Crystal.
Go north to the previous room, and go west to the middle of the room and go north through the wide door.
Get a Spell for Arche
There is a new spell for Arche that you can get here, or you can wait until after you fight the boss in this tower.
To get the spell, go north up the stairs, then go north up the hallway, then go west to the next room.
Make sure everyone is equipped with Nymph's Rings (as described earlier). If you don't have Nymph's Rings for everybody, be sure to keep an eye on everybody's health and heal them up as needed. Then go south to reach a lava room. Walk across the lava to get the Verbena, then go north to the previous room.
From here, if you go north and through the door, you will find two chests. The one on the right contains an Ankh Shield, and the one on the left contains a Flame Mantle. After you get these items, exit the room.
Use the northernmost east exit.
If you need Mixed Gummies, go through the door across from the stairs, get the Mixed Gummies, and exit the room. Otherwise, just go north up those stairs.
There you can optionally get a Paladin Helm, Life Bottle, and Shield Ring. If you don't want these treasures, skip to the next paragraph. Otherwise, if you do want the treasure, go north twice, then exit west for the ?Helm, then exit south and through the door directly south of you to get the Shield Ring from the chest on the left and a Life Bottle from the chest on the right. From this room, exit east, then go north up the hallway and go east, then go south down the hall and exit south.
In the lava room, use the southwest door. Go north up the stairs here, then go north through the hallway and exit east to find, from left to right, a Veal, another Veal, and a Holy Bottle. Exit this room.
Go north up the stairs, then continue going north and up the stairs in the north wall in each room until you reach a chest, which contains the Explode spell. Be sure to disable it.
Optional: Exit and Heal Up
You might want to leave the tower to heal up and buy supplies. To leave the tower from where you got the Explode spell, just go south and follow the path until you are in the lava room. Go down the stairs in the lava room, then go down the southwest hallway and go down the stairs, then go south until you are out of the tower.
Go to the Boss
To reach the boss of Odin's Tower, from the tower entrance, go east, past the hallway, and go through the door to the right of the hallway. Go up the stairs.
Open the chest to get a Reverse Doll. Exit west, then go south, and go up the stairs.
Exit north. The door here is locked. Exit west. Go west and south, and go through the first hallway on the left.
Flip the switch, then open the chest to get a Mental Ring. Go back past the locked door and exit east this time. Now go through the first hallway on the right.
Flip the switch, then open the chest, which contains an ?Armor that becomes a Star Cloak.
Now go back to the door, which is now open. Go through it and go up the stairs.
Go north from the stairs and go east. Pull the statue south until the door opens. Go through and get the Cute Mittens, which Arche or Mint can equip. Exit the room.
Now go west. Before the statue, there is a door. Go through it to get a ?Helm that becomes a Star Cap. Exit the room.
Pull the statue the same way as the other one, and go through the door that opens. Open the chest to get a Shield Ring. Exit the room.
Now go south and follow the path below.
Pull each elf statue south and onto one of the rectangular switches on the floor. Go north through the door that opened, and go up the stairs.
There are three doors in the north wall, and an exit to the south. Go through each door in turn, including the leftmost southern path, and beat the miniboss in each room. Each miniboss is just like a random encounter in the tower: two Karys, a Druid and a Djinn. One of the Karys will be behind you.
After you have defeated all of the minibosses (including the one to the south of the middle door), stairs appear in the north wall. Go up.
At the save point, walk around to heal up. Use Orange Gummys/Orange Gels/Orange Gummis on those with low TP. Set Arche to cast only Tidal Wave, and set Klarth/Claus to summon only Maxwell. Equip the two of them with Mystical Runes. Make sure Cless/Cress isn't using any fire skills. Set him to use Fury Slash for close range, and if he hasn't mastered Soul Edge yet, use that for long range.
When you are ready, go north.
Cast Haste on Cless/Cress and use a Flare Bottle on him. Have Arche cast Tidal Wave and have Klarth/Claus cast Maxwell.
Odin's Tower, Part 2
Be sure to re-equip everyone with Nymph's Rings. To leave the tower, go south to the lava room, go east and down the stairs, then go south through the door and exit east. Go north, then go down the stairs. Exit south three times, then follow the path, exit south twice, then go to the middle of the room and go south to leave the tower.
If you go to the Spirit Forest, south of Miguel, and approach the Tree of Life, there is an optional cutscene.
Go to Ymir Forest
Your next destination is the obelisk in Ymir Forest. If you are still at Odin's Tower, you can reach Ymir Forest by flying west. If you went to Spirit Forest, you can fly southeast.
Reach the Obelisk
At the Lake of Ymir, equip the Flamberge. You might also want to equip a Gorgon Amulet to prevent petrification.
After Arche leaves, make sure to equip Chester and put him behind Cless/Cress in the formation.
From the guard at the entrance of Ymir Forest, go north and then east across the bridge, then go north and follow the path until it splits. When it splits, go north until you can't go farther north, then go west until you can't go farther west, then go north to the next area.
Go west from the save point and follow the path to the next area. Keep going west, then when the path splits in four directions, go north into town.
Use the north exit of the Elven Village. To reach the obelisk from here, go north three times, east twice, north twice, east twice, south twice, then through the southeast exit, and east.
Get ready to fight Origin. First, set Cless/Cress to use Gale Shield, Lion Lunge and Mecha Blade. Set Klarth/Claus to call many spirits but only use Volt, and equip him with the Sapphire Ring and a Mystical Rune.
Go north to the obelisk to fight Origin.