This achievement is really simple. All you need to do is combo enemy combatants until Ryu sword looks as if it's stuck in them. Normal enemies will be simple to slice through will little to no effort, but later in the game you will meet different foes you will have to press the "Y" button to perform the move.
The achievement should be gained no later than the third level if you've been performing the action since the start of the game. If you have any questions, comments, or updates be sure to note them and I will update this guide.