Note: This game is also titled Gouma Reifu Den Izuna.
Successfully complete the game, then touch the crystal that Izuna touched at the start of the game. You will be at level 1 with no items in the Path Of Trails bonus dungeon. It has 100 floors and a secret Boss at the end.
Once you get to the Fire God's dungeon, you can continue to upgrade your weapons without stopping. Simply fight inside the Fire God's dungeon and find several burn-in orbs. Fill up your weapon with Zoufuku Talismans (increases the SP capacity of your weapon; the Zantetsu sword that ignores defense is recommended), then burn them in with the burn-in flame. Your increased SP capacity will remain on the weapon. After that, keep filling your sword with Rekka for attack, Mikagami for defense, Kaishin for critical hits, and Mikiri for evasion. Keep burning them in and switching to Zoufuku for more SP, and your sword will have very good stats by the time you finish. Make sure to keep an un-burned Kikan talisman stuck to it, just in case you actually are defeated.