When fighting in career mode or in a play now or
hard hits match, make sure your boxer has good
speed, power, and stamina. When the match starts,
only use haymakers as an attack. Eventually, your
haymakers will get more powerful and tire your
opponent down to create quick and easy knockdowns
leading to an easy knockout.
After you pick your fighters and finish training, at the arena screen, hold the control stick up until you hear a bell. Doing this will make your boxers about half the size the would regularly be.
Go the game modes and hold the control stick left until you hear a bell. You'll have unlocked all the fighting arenas.