Start by going to the Olympic Games and enter Olympic Mode. Select the King of the Arena event on High difficulty. Follow along through the tutorials until you get to the actual event.
This event is basically just a big battle, but instead of fighting your opponent, you are fighting a bunch of infinitely spawning Roman soldiers. You get points for hitting Romans soldiers, and whoever has the most points at the end of the match wins.
The key to winning this event is to use your Y,Y,Y attack to grab a stunned Roman, then spin him around and hit lots of other Romans, then throw him into a group of Romans to net yourself some massive points.
Be careful not to fall of of the platform, as this will cause you to lose the event.