This article is from the Final-Fantasy and other Square Soft Games FAQ, by email@example.com (Nick Zitzmann) with numerous contributions by others.
Try casting 'Freeze' on them four times (no, you can't use Quadra Magic,
but you can Mimic the spell three times afterward if you want to
conserve MP). They'll solidify and die after the fourth casting of
(NOTE: This trick only works in the Japanese version of the game. It
does not work in the USA version.)