Ok, you have just a few turns to beat him. Use beserk on Cecil ,only. Edge should throw
fuma shurikens. Make rydia cast Bahamut, and Rosa should just defend. If you time it
right, while odin is about to kill everyone with one attack, make Kain jump. Ihope this
will help you. Good luck.

use quick is the best way and if u have dualcast equip it than use ultima to defeat him