First go towards the Mercenary HQ, before you get there search for a bunch of thieves to hire (it must be thieves or this won't work).

After you have hired them, free-run to the fight club. Don't go through the door though! Make sure you scale the walls to get there!

When you get there wait for your thieves to stand beside you then choose to fight 5 people and bet 500 gold.

When you enter the ring your thieves will be in there too and kill the mercenaries, all you have to do is stand out of the way while they kill them (or you might get hit with a dagger) or you can help fight them.

But you get 40secs to beat them all up and it's done easily within that time.

When you are done you receive 1150 gold.