Johnny Charisma must be stopped in the course of the main story. He is one of the Joker Infected whom you have to apprehend once more during one of later visits in Panessa Studios. He is hiding in the sound stage C. This enemy has no health bar and beating him does not take a standard fight. Instead, you have to use a trick to catch him again.
Enter the stage where Johnny Charisma is. At this he starts singing as the Joker. The stage itself starts rotating slowly. You cannot attack Johnny as Batman, because he has armed five bombs and is wearing the sixth one. Instead, take control of Robin, who appears in the background. As Robin reach the first bomb, hide behind it and hold the correct key/button to disarm it.
Now you have to move Robin to the right. It is VERY IMPORTANT that you leave cover only when Johnny Charisma is facing you away, so he did not spot Batman's ally. Each time you get to another bomb hide behind it so not to get spotted while disarming it. There will be no way of hiding in the case of the third bomb, so you need a good timing to disarm it and hide again before the stage makes a full turn.
After disarming all five bombs take Robin toward the stage where Batman and Johnny Charisma are. Sneak up to the villain from behind and press the Silent Takedown key/button. Robin will disarm the bomb that Johnny Charisma is wearing and then easily stun him.