You have to either complete absolutely everything, or easier - Attach the nunchuck and press C, 1 (x3) Z, 1 (x4) then Z and c together and then 1 (x5)
In that order.
Hope you get it!
Also that gets you:
Completed game (straight away!)
White Mage
Ninja
Black Mage
Metal Slime
She-Slime
Other colour Yoshis
Cactuar
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