I'm fighting her too, and I can't beat her!!! It's makin' me crazy!!! I can knock her down over and over, but she keeps recharging. If you find anything out... Let ME know!
If you are on the second part of the battle, and it sounds like you are, go into the circle on the ground and hit L.
I was also stuck on this part, thought I'd never defeat Persephone, thanks for your help!
I had the same problem. But eventually I went to IGN and looked at their guide. It makes me wonder; do the developers think that people could beat games without guides. I consider myself a pretty intelligent person, but there were a couple of puzzles in this game I'm not sure many people are going to figure out unless they go into forums or different a gaming website.