I also found Myria to be the hardest boss in the game. I used Gau (Ninja rage - damage, protection from Zombie), Setzer (Seraph - Rerise and protection from Zombie), Strago (double casting Dark, wearing a Ribbon), Gogo (Slots for heals, magic for Rerise, Tools for defibrillator, wearing a Ribbon).
I can't imagine beating her without Celes/Setzer(Seraph) and Gau to protect from Zombie - I only had 2 ribbons for my other two party members, and I wasn't about to trade ??? for a third one. I originally had Cyan in this party instead of Setzer but I left and brought him in to help.
I went on to beat the other Triads and Kefka on my first try.