When creating a character, set his heart, power, and speed high. It is important to have the heart high. If you jab a lot then throw a punch, it will work much better than just bashing the buttons.
After completing the game you will be prompted to select a cheat. Choose teleportation. You will have to press your special and R1 to use it. If you are playing two player or exhibition mode, set the match to one round. Hit your opponent a few times then keep teleporting for the rest of the round and you will win. When creating a boxer, give him the Body Blaster as his special move. During the match, throw a few jabs and back away. Wait for your opponent to move in and hit him with the Body Blaster. It should stun him. Hit him with it again. Continue to do this and you will knock him out in no time. This can be done in all three weight divisions and works almost all of the time. Also, if you find yourself in trouble, try the Backhand (elbow) followed by the Body Blaster. It either takes off a good amount of energy or stuns them, leaving them open for the Body Blaster.
This is the easiest way to beat anybody. Get Tiger Lee in the Lightweight section. This is the beauty part, in a fight continue to use his Special which is R1 button. This special has a good advantage to all boxers, it's a 4 punch combu that if you continue using it through out the entire fight you'll be almost untouchable because the combo involves a back duck. Once your opponent is in the right position about to punch you you use the special and you can continue this and your opponent will not be able to block it %80 percent of the time.
Each time you beat the game with a CREATED CHARACTER you will unlock a cheat. To beat the game all you have to do is win the title and defend it 5 times. Once again, this only works with created characters.