Mistress Li was dealing with someone who did not want to give her the info that she wanted from him. So she felt that the only way to get it from him was to make him feel pain. And according to her, there was no better way to make him feel pain than to fist him. So she did not hesitate to torture as well as to dominate him cruelly.
Mistress Li did not want her competitor to use unfair means to gain advantage. She was ready for a fair fight but when she realized that she was being subjected to a rigged game, she had to act since the guy had already skewed things to his favor. The mistress chose to fist the competitor and let him know that he would have to deal with her directly in the event that he cheated in any way.