For those of you who have played this series before this trophy will come as no surprise.
"Over 9,000?!" - Dealt over 10,000 damage to an opponent in a single combo
As the icon demonstrates, we'll want to use Hakumen in order to obtain this trophy. The opponent will be Carl clover on a second controller or a friend online. At the start of the battle, drain Carl's "Barrier gauge" by holding back and Square+Triangle. You should have a shield up and the barrier meter should deplete. Get it close to empty, then begin to back up and advance with your primary controller. When you get to the wall, jump over the player getting the trophy and keep backing up. Make sure your barrier meter stays low while doing this.
Once you see red lines surrounding Carl he'll be under the ailment of "Negative Effect". This won't happen unless you're actually backing up for a decent portion of time. Backing up against the wall doesn't work. Anyway, this will greatly lower his defense. Once this happens, use the rest of your barrier gauge as well which results in an even greater defense reduction.
With both of these criteria fulfilled you can now use Hakumens distortion (Half circle back, forward + Circle). Make sure you charge the distortion as long as you can and make sure the other player is right beside you so the brunt of the attack hits him. If done correctly, you will do 10,000 damage with this one swing.
NOTE; If the input for the distortion is annoying you, simply use "stylish" mode and hold Square once you have 4 meters full as Hakument and the criteria above are completed. It's a bit simpler. You still must hold Square for the maximum damage.