Very tedious achievement, here's how I did it:
-Set the score to 99
-Loaded up 4 controllers
-Played on the Sushi Bar map
I walked along the bar waiting for the right sushi to roll by while punching everyone in sight. The logic here is once you have a full sushi order you'll get 2 points plus any points you managed to get by KO'ing any of your dummy accounts. An optimal run would be 4 points: 2 for collecting all the sushi and 2 more for knocking out two of the dummys.
If you do happen to get two KOs, stop punching because if you knock out the third dummy the match will end immediately and you'll lose the 2 points for getting all the sushi.
This method is a minimum of 25 games played but I think it's way faster than anything else I was experimenting with trying to quick grind this out.