How do you fight Zelos in Tales Of Symphonia?

Answer 1

First, you equip all your gems of every kind to everyone in your party, then you go behind a house in the floating village of half-elves, there is a hidden path, and this has to be after all temples, and before the final battle as well.

Answer 2

During the scene in Flanoir, after you get a doctor for Altessa, choose not to talk with anyone. You will then enter a conversation with Kratos. Later on, you will Fight Zelos, and even later still, Kratos will join your party.