Ready 2 Rumble Boxing

In the Arcade mode, go to the boxer you want and press c left + c up at the same time

Before you select your boxer in arcade mode, hold down the Left High Punch button. When you push the D-Pad in a specific direction, you'll choose 1 of 5 cornermen. If you hold down the Left High Punch, Low Block, and High Block buttons, you'll get a high pitched version of the Irish guy. He sort of sounds like a leprechaun.

From the main menu, choose Championship Mode. Start a new game, name your gym, choose a boxer and pick the Train Boxer option. Now highlight Rumble Aerobics Training, press Left, then quickly press A. If you did it fast enough, you will purchase the Rumble Mass Nutrition Regime (which costs $25,000) for $500! This will boost your strength and experience very quickly. You can also purchase the Sway Bag for $500 instead of $1,000 by pressing Right instead of Left.

Press C-Up + C-Down + C-Left + C-Right repeatedly until Tank Trasher begins to make fun of his opponent.