If you want to humiliate him, diminish his worth in front of everyone and simply act like you are above him—the same way he has always been acting toward you. This will never have the same effect unless you do it publicly. No narcissist can stand people looking down on him or ridiculing him and that is exactly what you should accomplish if you want to really put him down.
This will make him more insecure than he really is and it will make him question his every move and decision. It will lower his self-esteem even more and it will make him paranoid. Of course, every time he accuses you of trying to do this, make sure you convince him he is imagining things and that you are only trying to give him some advice for his own good.