Start by casting Slow with Rosa, then have her cast Haste on everyone. Edge should throw any Fuma Shurikens, followed by normal Shurikens (if you didn't buy any in the Eblan cave, shame on you). Cecil should attack normally, Kain should use Jump, and Rydia should alternate bewteen Bluff (DS version only) and summoning Leviathan. Even if you don't defeat it before it gets close, Slowing it down gives you enough time to use a Phoenix Down and still get several attacks in before it uses Crush again.