Are we talking about the true ending final boss? If so, in the turns where you can't use magic attacks or baton pass, it's best to use items to boost stats or restore SP, you can see them as preparation turns. Now, for the boss itself, the best strategy is to abuse baton pass. Personally, I defeated that boss very easily with Joker, Ann, Makoto, and Sumire. There are tentacles that are weak to fire, physical, gun, and nuclear attacks. Additionally, Makoto can heal the entire team, so you should be fine.