El Shaddai: Ascension of the Metatron

Once you are on Armaros' floor, you will enter an amphitheater type place where he puts on a dance performance while these little grunt back-up dancer guys keep climbing up from the back of the stage. Until he finishes his dance (2 minutes or less it seemed) you can keep killing these little fellows. If you are up against the edge of where you can walk to, they will hang back and not come into range if you are armed with the veil or possibly the arc - I would recommend trying to keep the gale on hand once you are on Armaros' floor, as this seems like the best way to kill these guys, and, although they don't have much health even on normal, obviously they'd die faster on easy difficulty. The last Lucifel save point before this event where you can change weapons is the one with the lake with islands floating by and Lucifel is saying "Yeah, we're still underwater." I killed far more with the gale than when I initially tried with the veil which I came into the "show" with, but I was still not able to kill 50 in time after a couple tries. I think I got pretty close, but it seems like this would be most easily obtained, obviously, on easy, and so far I am only on my first playthrough on Normal.

Edit: After completing the game, I went back to this stage using chapter select and put it on easy, and killed 70+ of the little dancers using the gale. You can even save up an Overboost for this section if you want, but it's not even necessary using this method.