Go to the options menu and set "Illegal Blows" to "Off". Then, purchase the illegal head punch called the "Elbow Shot" at the fight store. Start a fight and use the Elbow Shot to counter attack each time your opponent throws a punch. Note: it is important you use the Elbow Shot as a counter attack so that you can connect cleanly and powerfully on you opponent's head. You will see that the Elbow Shot inflicts a lot of damage to your opponent, and will put him KO in a short period of time, or TKO because you inflicted so much damage to his eye.
Go to the options in career mode and set the H.U.D. on and turn off illegal moves. During the fight only perform knees to the stomach. Note: You buy the knee move in the Fight store under "Illegal Blows To The Body". You can see your opponents health go down and then it will go into Knockout mode. Then, knock them out. Knees make your opponents health go down faster.
When facing a boxer that is stronger than your fighter, try hitting them and moving away. Throw a jab or two then move away from them. This works very well if your boxer is faster. This also helps if your boxer is a lower weight class.