It's easiest to try this in a fight with a regular enemy and not a boss battle. Wait until the enemy attacks and use RT to deflect his attack, after that immediately start a combo to get the enemy towards the edge. If you can knock him off the edge at the end of the combo the achievement will unlock.
If you did not unlock it early in the game, you will automatically get it after the boss battle in the throne room (near the end), because you can't hurt the boss there, so the game will think that you defeated the boss without attacking first.