Prison Break: The Conspiracy

The "Unbruised" achievement is not hard at all, and can be aquired in Chapter 7. It's better to have your fighting skill at the "Untouchable" level, so if this isn't the case, just lift some weights or hit the heavy bag while in the main yard to do so.

Once you're ready go find Mr. King in the main yard and he will set you up with an underground fight. Choose Soda Pop as your opponent. When Soda Pop begins to throw a punch tap the "Right Bumper" (RB) to counter. If successful, you will be prompted to tap either "A", "X", or "Y". Be sure to kick your opponent if he's knocked down by tapping "A". This will deplete his health a lot faster. Once his health is low enough you will be prompted to tap "Y", which will perform a finishing move and end the fight.

Another method is to begin the same way as above, but instead of using the counter/reversal method as previously stated, as soon as the fight begins just tap "A" repeatedly to throw a flurry of punches at your opponent. Continue to do this and you should win with ease. Remember to kick ("A") him when he gets knocked down. You will also be prompted to tap "Y" for the finisher when your opponent's health is low enough.

The counter/reversal method worked best for me, but both work with minimal effort.