Once you reach the Fire God's dungeon, you can continuously upgrade your weapons. Fight inside the Fire God's dungeon and find several burn-in orbs. Fill up your weapon with Zoufuku Talismans, then burn them in with the burn-in flame. Your increased SP capacity will remain on the weapon. Then, keep filling your sword with Rekka for attack, Mikagami for defense, Kaishin for critical hits, and Mikiri for evasion. Continue burning them in and switching to Zoufuku for more SP, and your sword will have very good stats by the end. Make sure to keep an un-burned Kikan talisman stuck to it, just in case you actually are defeated.