Cynthia by a long. long shot. It was the longest, hardest and luckiest battle of my life.
Lasted nearly 1 hour and 20 minutes and involved luck, openings and plenty of Revives and Full Restores. And I had a party about 15 levels down from Cynthia's. Gotta give it all to the Amulet Coins + Rich people tag teams in the Valor Restaurant (for allowing me to get 70+ Revives and Full Restores).