Fighting Zeto can be rather easy with the right equipment. Zeto has a multi-hit physical combo that can hit an un-Spirited character for over 100 damage each strike. He also sports an attack called Poisonous Bubbles that hits for slightly over 100 damage and can inflict Venom on a single character. His multi-target attack is called Big Wave and it can kill the whole party if they are not spirited. However, this is not a worry due to the attack requiring a charge move. Before Zeto casts Big Wave he will always use the ability Call Wave, and the message "expect a powerful attack on the next turn!" will show as a warning to have everyone Spirit. With Spirit enacted, everyone should take slightly over 100 damage.
Equipping a Deep Sea Jewel on everyone will make this easy battle even less difficult as it will reduce damage from all of Zeto's attacks. Aside from that, accessories do not matter a whole lot in this battle. Every time Zeto casts Big Wave on the party, having one person use a Healing Bloom will completely recover all damage received. Just attack with everyone consistently and Spirit with everyone after Call Wave and the battle will be over before long.