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.