Complete the level 3 bets at the PMC Villa with your allies. These unlock extra costumes for your mercenary. For the best results, bet the maximum amount of money for the first two levels of challenge (manually save and reload your game to insure you win). For the third level of challenge, bet the minimum amount of money -- you will need to be perfect to beat a challenge at level 3.

Note: you can also have a second player (online only) to assist you in unlocking the final challenges. In such a case, both players should receive the costume (remember to save if auto-save is turned off!)