This only works after you take part in the Lacour Arms Tournament.
You must fight Dias to be able to do this also, Just go visit Gamgee after you lose to Dias and he'll give you the sword called 'Sharp Edge'.
Keep this sword around, and collect 2 Mithril. Make sure you have the Customization Skill and Orchestra Super Specialty Skill at level 10, Use Orchestra and bring up Claude's Customization.
Use the mithril on the Sharp Edge to make the 'Minus Sword' then use the second mithril on the Minus Sword to make the 'Eternal Sphere' this sword has 1600 attack, and throws out stars when it is swung, which knocks enemies back and is most useful against those Shield-Bearing enemies.
We have more cheats and tips for this game here, Star Ocean: The 2nd Story cheats
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