The easiest way I found to do this was to set up a Vs Battle against a second controller. Put the second controller playing as Heihachi, the other character doesn't matter. When the match starts run Heihachi towards your character (by double pressing
) and press Kick (
if using default controls) when he's near the edge of the stage. He should do a slide kick past your character and straight over the edge.
By using this method you should easily get the achievement within 2 - 3 minutes.
*Note: This will also work in online battles.