This can be a real pain in the ass battle if you dont know what you are doing. First off before you head down to the bridge where you face odin you need to use two shrouds Aegisol and Fortisol
If you do not have the shrouds the you will need to make a paradigm with lightening as a medic and hope as a synergist (known as symbiosis) and set it as your default. Now start the fight.the way to beat odin is to heal while increasing his chain guage. if you dont have the shrouds on then keep hope healed untill he has cast protect and shell on both party members withut these you will not survie his attacks. I had the shrouds tho so the vid did not show that step.
once hope has cast the buffs change to War and Peace this enbles lightning to start building up the chain guage while hope does some vital healing. watch odin carefully and as soon as he casts ullr's Shield switch to dualcasting and lay into him building up the chain guage the moment he resumes attacking switch back to war and peace keep this going untill his guastat meter is full and hit x to claim victory (square on ps3) i hope this helps