This unlocks after achieving 150 "EXCELLENT" sync impact rankings throughout your playthroughs of Asura's Wrath.
Due to the number of playthroughs required for the completion (A minimum of 3 not including DLC content) you'll earn this naturally as you get familiar with the timing window. The timing windows seems to be directly tied to the animation happening in the background as well as the circular cue in the middle of your screen so I found that sometimes using the animation to help guide my timing helped out tremendously. Great sync rankings typically meant that I was a little fast and needed to wait just a bit more (like .5 seconds).
If you really want to grind it out I would recommend replaying Episode 15 on Easy as there are 10 opportunities to earn excellent sync rankings and only 2 fights that you'll need to do. You can skip cutscenes with the
button too, although not all of them are skippable