The strategy guide has these steps listed, but has a couple of errors in it. I am updating them as I confirm these errors.
(2 known errors in step: 2 only available after getting 4 spirits... aer krene is SW of Nordopolica)
Here's the final edited version. I've added some more detail to the less specific steps, and removed spoilers.
Be warned that if you complete this, you will be forced into an extra fight on the last boss, and he is far more powerful than normal players will be prepared for at that point.
1. Go to the room of the Myorzo Elder and acquire the Abyssion sword. This can be accomplished once Zaude appears.
2. Use Ba'ul to reach the aer krene to the SW of Nordopolica and approach the boulder blocking the entrance to receive the Zarich. Must be AFTER you get all 4 spirits.
3. Get to the bottom of aer krene to level up the sorcery ring to lvl 4.
4. Get the fell arms Nebilim from the treasure chest in the furthest map in the forest of Keiv Moc. Burn the bushes that block the path.
5. Get the fell arms Uroborg and Susanoh from the chest in the final dungeon.
--a. You may find the following steps easier with the Sorcerer's ring lvl 5, but only lvl 4 is required. to get level 5, go to any other rock formation aer krene in the world, and it'll level up in the spring.
--b. Fight your way through the dungeon up the tower to the stone cavern. Activate ALL blastia here and then follow the path east. You will be outside the cavern and at the stairs of the tower again. Go down until you are back at the blue stairs area again.
--c. follow the short path, across the air bridge, to a glyph, which you can walk across to get to a new area with ladders and platforms. Solve the simple puzzle without too much worry.
--d. eventually you will find another glyph. Follow it out of the ladder puzzle temporarily and you will find yourself in the blue stairs again. Go through this area and you'll find yourself in a red stairs area. The two fell arms can easily be found here.
--e. This would be a great time to backtrack to the caves and escape the dungeon. Go up the stairs near the final save point and you will be able to go to the left to an elevator, which will unlock all other elevators. Take it to level 1 and get on Ba'ul.
6. Once you have at least 4 of the fell arms, an event will occur when you board Ba'ul.
7. Defeat the Chimera Butterfly in Quoi Woods (if you haven't already), then get the fell arms Glasya Labolas from the chest to the left of the tree. You need to burn the brush (with the lvl 4/5 ring) near the north entrance to reach this area.
8 Get the Magic Sword Mercurius from the chest by going down the lower west road at Relewiese. Just go down the hill southwest of the healing savepoint and the return warp. It's a short path to the sword.
9. Once you've collected 7 magic swords, an event will occur when you board Ba'ul again.
10. Return to Auronion. An event will occur when you enter Auronion. (If another event occurs instead, leave and come back)
11. When you get to the last battle with the final boss, an event will occur wherein you unleash the power of the magic swords. This will happen after you defeat his second form.