If you are having trouble completing a fighting mission where you must fight thugs hand-to-hand, it is best to drive around to different Dojos when not on a mission to learn different fighting styles. Some styles are better used on some opponents than others. The best styles are Thai Boxing and Tae Kwon Do. They seem to be effective on a wider variety of opponents and can defeat more fighting styles than other ones. Also, do not overlook your default "Brawling" fighting moves as they can prove very effective against some of the fancier martial arts like Wushu. Switch between styles while in combat to find the best one for each situation. If you can score hits with combos of three or more moves on one opponent without them blocking any of your hits (or stunning them), then your style is better than theirs. Also, during the "fight club" mini-missions you can sometimes get a hint as to what styles your opponents are going to use by the little gestures and moves they make before the beginning of a match.