Get Selene
Collect all nine Sages and go to the Moon Palace. Use each of the Sages' powers to break the spells. Then, equip the Moon Sword on Arty and use all of the Sages' Sword Spells. Go to the gate and fight Earth Seed. One hit from the sword should destroy him. Make sure Arty is at level 65 or higher, and use Cele's Pearl Fist if necessary. Talk to Luna to open a portal. Then, use the Hero Stone and Luna will become Selene. Her spells are Moon Sword, Energy Lift, and High Sky, and are very powerful.