Once you complete the mission that unlocks the mercenaries you can find the fights in the basement of Bartolomeo's Barracks (where the mission started).
If anyone is having trouble dispatching the opponents in the time limit i have a way you can try.
The easiest way i found to eliminate the opponents is:
- When the round starts hold
when they are close enough to stun them (they should have a red outline)
- Choose one guy and try
for an insta-kill (if it doesn't work doesn't matter)
- Then proceed to concentrate on them one at a time using
to break block then mash
until downed he should be downed in around 4-6 hits after the block break
Using this method i had 14 seconds left when i finished the last guy in round 5
You only need to bet 500f on the ( 5 Mercenaries Fight ) to get the achievement
On all other fights you don't need to bet anything unless you want to.
Hope this guide has helped at least a few of you