This achievement can be done pretty fast, providing that you have a boosting partner (or if you have a second xbox + copy of the game).
First, create a lobby and set the time limit to 30, the life bar to smallest. Rounds to 1, and most important: LOSER STAYS.
Why loser stays? It's because the challenger gets to pick the map. And that makes it easier when one player is AFK.
Always pick the map Dojo, it has the shortest loading time.
When none of the players is AFK this is what you do, given that you started your lobby with the setting listed above. One of you attacks the other with something like p,p,p, so you make sure you do not knock the other player down. It saves time! When someone has to stand up all the time you're losing seconds. You can easily finish rounds like this in < 8 seconds. Add some time for the loading the map, picking costume etc and you're looking at ~40-45 seconds a match.
When one player is AFK it only adds 20 seconds per match so you're looking at ~60-65 seconds per match. This is because of the loser stays mode. The player who isn't AFK can select the map so you won't have to wait the 20 seconds at map selection.
When BOTH players are AFK you can just let the time run out (30 seconds). First match will be a draw, second match the defender will win. So this will take you 30*2 + 20 + 20 + loading = around 2 minutes per match.
All in all this won't take you too much time. It's just pretty boring.
Just a side note; it doesn't matter if your controller disconnects while being AFK, the game will still run.