This Trophy comes from Natural Progression
No doubt you will perform more than 50 All-out attacks over the course of the game. To perform one you must either exploit an enemy weakness (casting Agio on a enemy weak to fire) or perform a critical hit which results in the enemy getting knocked down onto the floor. Once all enemies have been knocked down, whether that's 1 enemy or 5 enemies a cue will pop up to perform a group attack where everyone in the party rushes in and does massive physical damage to all enemies.
Perform 50 and this Trophy is yours! For reference sake, i obtained this before the end of the second dungeon, steamy bathouse. Will differ between players though.