If you still can, cast Bersk on Cecil, possibly Edge as well. Use Lit3, or maybe even call Bahamut if you have him, but I don't know how much damage the lightning-elemental summon would do (Indra? Or was it Ramuh?). Rosa's attacks wouldn't be able to contribute much, unless she knows White (Holy), so I'd just have her Haste-ing and Bersk-ing people.

If it's timed right, Kain can be jumping and in the air when Odin does his attack, and will live through it.