In the options (press Select) set number of rounds to 1 and when selecting opponent set AI difficulty to 'Very Easy' (can also use a second controller if available).
Hit the enemy until their health bar is drained at which point 'FINISH HIM' will appear on screen. Position your character midway from the opponent and input 'Down + LT' four times to perform a Mercy. At this point you can finish off your opponent.
Then when the points are tallied hit 'Retry' to restart and perform 9 more Mercies.