This trophy is titled "You're the best! Around!" and it's for dishing out 1,000,000 damage in practice mode. The best way to work towards this in a fast manner in my opinion was setting the distortion drive to maximum, turning on the negative penalty and flicking left on the right stick using Hakumen and holding circle to charge his distortion to it's maximum damage.
I'd only suggest doing this after knocking out the two combo challenges related to both Hakumen and Nu. You'll do some decent damage while working on earning these trophies in practice mode, though if you don't plan on earning these trophies or simply want your quick fix, the method above is the way to go.