Zenobia is initially immune to damage because she is covered by a wall of tentacles. The player must defeat the tentacles to be able to hurt Zenobia herself. Once Zenobia's protection is gone, the party should switch to Tri-disaster (RAV/RAV/RAV) to build up her stagger meter to 999%.
Zenobia will raise her shield again once her health is low. At this point, it is good to switch to a defensive paradigm like SAB/SAB/MED. Zenobia will hit the party hard with a physical attack that can knock the player out if not prepared, but after surviving this assault, the player should go back to taking out the tentacles before finishing off Zenobia followed by a Cinematic Action sequence.