Prison Break: The Conspiracy

You can access the underground fights as early as the first chapter by talking to the character "King", he's to the left of some workout benches, you'll need to have the achievement "Quid Pro Quo" before King will allow you to participate in the fights.

I suggest leaving this achievement for your Shark playthrough if you intend on doing one as there's an achievement that requires you to beat up 100 people on Shark and you can nail this at the same time as that.

The quickest way to get 50 wins (Or 100 if you followed my advice!) is to choose Soda Pop as your opponent, he is the slowest and weakest of the lot.

Here's a video giving you a general idea of what to expect.

*^{Credit to lordGTAwarrior for the video}

This achievement can be aquired in Chapter 7. By this point you should have easily built your fighting skill up to "Untouchable". If not, hit the weights or the heavy bag in the main yard to do so before attempting the achievement. Once your fighting skill is maxed, find Mr. King in the yard and he will set you up with an underground fight. Choose the first opponent, Soda Pop. Choosing Soda Pop will allow you to complete this achievement rather quickly and easily.

There are a couple of ways to go about defeating Soda Pop, and depending on which difficulty setting you are playing through, this could net you some other achievements in the process:

1) Unleash a flurry of quick punches by repeatedly tapping the "A" button. Continue this throughout the fight. With Soda Pop's lack of speed, he will almost never hit you.

2) Wait until you see Soda Pop about to throw a punch and tap the "Right Bumper" to counter his punch. As this happens, tap the corresponding "quick-time" button that appears on the screen. Repeat this until his health is depleted. Again, you can very easily defeat him in seconds with getting a scratch using this method .

When his health is low enough, you will be prompted on the screen to tap "Y". This will perform a finishing move and "BAM", the fight is over!

Either way you go, be sure to kick your opponent when he's down (literally!). To do this, walk up next to him when he is on the ground and tap "A". You should be able to get one or two kicks in before he gets back to his feet. These do a tremendous amount of damage to the opponent!

Just repeat this 50 times to unlock the "Ringer" achievement.