This is actually earned by completing all other achievements.

Defeat Klinsy the librarian and obtain the Dia Missile spell. Use the spell in an area of any level without any enemies in range. When used while the crosshairs are not visible, your bonus gauge will fill a small amount. Repeat the process until the gauge is filled, allowing you to get any desired upgrade.
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Complete the game as Zizou.

This game is titled The Legend Of Dark Witch in North America and Europe, and in Japan.