Your best bet for this trophy is to take someone that is a combo beast into Beat Attack Intermediate. Here you have nothing but those annoying hollow heads flying around, and 3 minutes to take out as many as you possibly can.
The following characters do extremely well here after ~level 50:
Stark (standard square combo)
Final Ichigo (triangle moves, and his circle move in the air for higher combo)
Ishida (triangle moves, mixed with his circle move when the vials are full)
Nnoritra (straight triangle moves)
Aizen (spamming jump + circle)
So far I have been averaging anywhere between 350 and 400 hollows defeated every time I enter this mission, with anyone other than Aizen. With Aizen, I can get 500+ every go. If I were to take worst case scenario:
33,650 / 350 = 96.1429
I know doing that mission 97 times seems like a nasty grind, however you can easily knock out some max levels here too. I have been doing this with Nnoritra since level 30, and in 1 hour (approx. 13 rounds) I had him at ~level 120. Every round I was earning around 3.2k of soul points. Same with Stark. But Nnoritra really needs spirit upgrades to do the most devastation, so focus on those with him to get the most bang for your buck.
Ichigo can be a bit more difficult if you use the circle in the air move for a high combo, as you waste time and don't add a lot of kills in the mix. If you stick with straight triangle attacks without the spirit regen, and extra spirit bars, then you may come up short as well.
Same with Ishida, since most of his range, and devastation, comes from his triangle moves. However, Ishida is unique in that he also has 5 vials that fill up giving him an ability to use other techniques (Ginto) that have range which do not consume spirit. And when you activate Ishida's ignition, those vials fill up as fast as you use them, so if you are having difficult managing your spirit gauge, use that circle attack (on the ground only!) and let your spirit bar have a chance to fill back up.
Aizen is the easiest to use here, and really maximize kills. His jump + circle move sends little orbs out that just destroy everything regardless of who you are focused on. I would just stand in the middle of the area and spam that move over and over again. You can also net a crazy amount of soul points for Aizen using this method - more so than any other grinding spot I have found so far for him (~3500 soul points a run).
If I missed anything please let me know in the comments below.