Once again you must talk to Centurion to start this fight. I guess you may be a little surprised to hear that Cusa is going to be a rather weak opponent. Keep blocking his attacks and perform countermoves when he tries to deliver the next punch [#160]. You'll probably need some time to practice, but when you get the hang of it you'll defeat him without any problems nor health losses. Centurion will give you credits and a cigarette pack (20 - Lensing Necro) for the win.
You must now decide how you want to proceed. Remember that there are two ways for you to gain access to the mines. The first method will require you to go to the Recreation Area. Proceed to section A and approach a smaller gate with two guards [#161]. Wait for a short cut-scene to be displayed on your screen. Riddick will be caught carrying drugs bought from Twotongue and this will result in him being transported to the interrogation room.
The second method will require you to defeat your final opponent in the ring. In my opinion this is a better outcome, because otherwise you wouldn't have a chance to collect one of the packs. Go to Centurion and you'll find out that your final opponent is a guard called Bam [#162]. Winning shouldn't be too difficult. Remember to keep moving. Attack Bam with swift blows and prevent him from making too many successful counterattacks. If you're skilled enough, you shouldn't lose more than one health unit. Wait for a cut-scene.