I think you need to keep your HP up at least. And try to use health restoratives as well. I'd recommend trying to equip Oak Ring, or another item or two that helps with magical defense.
It may be, and I may just be missing something. I don't think her dealing more damage than I heal is necessarily a balance issue. The balance issue I see is that whatever her health is, the bar doesn't appear to move regardless of how much damage you deal to her, which means either the bar is visually bugged or her health is insanely high.