This trophy is for defeating the boss of Calix VIII, the 8th level of the bonus dungeon in the post-game. The bonus dungeon is accessed by completing the game, loading up the clear game save file, and then going into the new warp in front of the final boss' room.
This fight is against Jade and Sophie from past Tales games. You must attempt to reduce their HP evenly and kill them at the same time because when one of them dies, the other gets powered up. Jade's powered up mystic arte is likely a one-shot death for the entire party. Sophie will just mow you down with melee attacks in seconds. Save up BG for when they're both low, and unleash mystic artes to finish it.
It's recommended you do the entire bonus dungeon on at least normal difficulty or else you won't be able to get the silver trophy: "The Sky's the Limit"