I noticed that what you do on looby (where you select number of players) counts to the achievement. So you can stay there punching lots of time to get the achievement. BUT... punch only counts if hit other player. I needed 2 controllers to keep punching second player against the wall. If you have a turbo controller,will be much more easy, just keep
preesed for a while (about 3 minutes). To bank the counter, just start playing a game.