Execute the Royal Retribution attack ten times to unlock the achievement.
The Royal Retribution attack is performed by executing a warp strike when the phantasm bar or sword gauge full and showing 12 swords. This is a purplish semi-circle in the bottom left corner of the screen. A warp strike is performed by pressing the targeting button and the warp button simultaneously. Using controller configuration A, this means you press
. If the sword gauge is not full, you will execute a warp strike. If it is full with 12 swords, you will unleash a barrage that will inflict a large number of hits.
Towards the end of chapter 2, you receive a tutorial on how to perform Royal Retribution including the buttons you must press according to your controller configuration. Following the tutorial you receive a challenge to inflict Royal Retribution. You have a certain amount of time to follow the tutorial's instructions in order to execute it. It is not difficult because the phantasm bar is already full. So, all you have to do is press the targeting and warp buttons simultaneously.
If you do this within the required time, you receive a 'success' notification. Whereas you must receive the success notification as part of an A+ ranking for successful strategies, you are not required to receive it for this achievement. The only condition for unlocking this achievement is to execute the Royal Retribution attack ten times.
The phantasm bar is easy to fill by performing combos or demonifying enemies. You will have numerous opportunities to use it. Since it is part of fighting effectively, you will most likely be using it dozens of times throughout the game and there is no need to grind it. After you execute the tenth attack, the achievement will unlock.