I realize I do not have this trophy, but I think I've figured out an easy way to do it.
In this game, enemies do not disappear after dying; you can continue to hit them. As such, Hibari is the right one for the job.
Simply choose an easy mission with many weak enemies, preferrably one in a close space.
Make sure to get the enemies close together, and then proceed to do her Airborne Square attack, which is her buttstomp. This is repeatable, and can be performed every (roughly) 1.5 seconds. The more enemies you have together, the faster it'll go. It should take anywhere from 30 to 45 minutes.
I realize this is a little long, but that's based off of a minimum of 6 enemies. Adjust based on the number of hits you're getting each time.