Make your party the 2-1 (Attack) Formation. Place characters as 1. Ein, 2. Cierra, and 3. Serene. Once the fight begins, keep flinging Einherjar at him until he changes to be weak against fire. He should be yellow or a similar color. Check his stats (press Start and select "Stats") to find out. Then, build your Overdrive bar to the maximum and use Cierra's level 3 Overskill to do damage him severely. Repeat this until Hector is defeated.