Assassinate him during the intermission

From Create Your Own Story

He turns around and slips behind the curtain. As the people around you begin talking to each other, you approach the stage and slip behind the curtain as he did. Behind the curtain are a door and a large chest of what you believe to be his magician’s supplies. Cracking open the door, you peer into the adjacent hallway and briefly see the magician walking down the hallway and out a door to his right.

Follow him and assassinate him
Place a mine in the magician’s equipment