If this is the "Talisman Class Secutor" (oh god that warning voiceover), then when the boss spawns the duplicates, and turns translucent himself, they shuffle around and spread out, if you play like a game of keep your eye on the ball, you can pay attention to which of them (the one that starts in the middle) is the real one, follow him, and just start beating away at him. As soon his shield goes down the copies will disappear.