True mastery is not what one can do to others. Rather, true mastery is to what degree one can control one's self.
The Greek word sophrosyne is exact definition of true mastery. To get sophrosyne one needs to defeat the opponent within. This is what O Sensei was referring to when he talked about Masakatsu agatsu or "The true victory is the victory over one's self." If you want mastery, journey inwards.