First off, you need to complete Sequence 3, Memory 2 to access the barracks. If you've completed the game or gone past that point in the game, you can find the Barracks of the right side of the map, as it will be indicated with a sword icon. Or you can just fast travel there. Anyway, once you've gotten to the barracks, go down the stairs and talk to Bartolomeo by pressing . Below is the table of general information for each fight.
 

# of M	Versus	Award Multiplier	Time Limit
1	Mercenary	1 : 1.1	00:10.0
2	Mercenaries	1 : 1.2	00:20.0
3	Mercenaries	1 : 1.5	00.25.0
4	Mercenaries	1 : 1.7	00.35.0
5	Mercenaries	1 : 2.0	00.40.0

For the trophy, you have beat the 5 mercenaries in 00.40.0 while betting 500f. But before you can beat the 5 mercenaries, you have to beat the other mercenaries. This will give you a bit of a preview of what you're in for. It'll take you a few tries but you will get it. Below is a recommended method to deal with all 5 mercenaries.

First off, run right into the arena and prepare to throw sand. Hold  and point the  stick upwards. Try to target as many mercenaries (The maximum is three) as you can. (If they are highlighted red, then you are targeting them.) Then release . The mercenaries you targeted should all be stunned. Attack one of the mercenaries. If Ezio performs an knockout animation, then you will have 4 left. (If he regularly hits, then attack the next one.) While in the animation, one of the mercenaries are going to attack you. Counter it ( + ) for an instant K.O. Three more remaining. Throw sand at a mercenary and then attack him. Switch between hits and kicks while attacking. If a mercenary attacks you, counter it. That should leave two more remaining. Finish off the mercenary you were attacking before, then do the same thing with the last mercenary. After that, he should be down.

This will take a few tries, but hopefully it works for you. After you win, the trophy will unlock.