The Gloves Come Off - Win the highest bet at the Fight Club
First you need to finish Sequence 3 of the story to unlock this event. Then the fights will become available in Bartolomeo’s house, the Barracks.
You don’t have to go through 5 levels of fighting, you only have to do the last level of the fight. Bet for the maximum amount of the fight which is 500f and then you have to win.
Finishing this will reward you with 10G for the XBOX 360 and a bronze trophy for the PS3.
If you are still having problems, you can watch the video here.