Easy achievement, took me less than 5 minutes. You have to finish 2 enemies without ever getting hit.
From the MAIN MENU go to
HELP & OPTIONS >
*Change CPU-LEVEL to 1
*Change DEFENSE LEVEL 1P to 8
*Under the last setting, set the P2 Defense Level to 1
*Set the POWER GAUGE 1P to MAX
Now go back to the MAIN MENU and select STORY MODE
At Character Select choose HAOHMARU (top row) and MUDMAN (bottom row, right corner)
At the VS screen use up/down on D-PAD to make sure that 1st Player is displayed above HAOHMARU
Start the match and pretty much just keep pressing the Y button. I stayed close to the opponent and just kept mashing. The achievement will pop after the 2nd opponent is defeated.
There is some minor luck involved in this, if you get hit at all you need to restart the match. Sometimes your opponents will be tricky, but they change every time you try. Just try this a few times and you should get it no sweat.