Each time you land a hit on a Roman your combo meter will increase by 1, you then have a short amount of time to land another hit before your combo meter goes away. Hitting multiple Romans at the same time will increase your meter for each Roman hit (i.e. hitting 3 Romans with 1 attack will increase your meter by 3). Your best bet to get these achievements are in places where an unlimited (or nearly unlimited) number of Romans are available, and using combo attacks like grab and twirl to hit large groups.
I would suggest doing this at one of the following places:
When the Romans are attacking Lovestorix's chariot and you have to throw Romans at the dart board to open the door.
When you're competing in the King of the Arena event (this is when I got it).
Or at the end of the game when you are fighting 1,000,000 Romans.