Better created character
When creating a character, set his heart, power, and speed high. It is important to have the heart to be high. If you jab a lot then throw a punch, it will work much better than just bashing the buttons.

Bernard Hopkins
Choose Bernard Hopkins and keep pressing R2. He throws a very effective uppercut and can knock out the opponent in two or three rounds.

Easy victory
When creating a boxer, give him a 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.

Easy win
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.