Every time she attacks, she loses health. She has a long catalogue of skills, with the HP damage increasing as you go down. However, since she's a ranged caster, she has a "Range" stat. So to use "Mana Strike" a melee attack, you have to actually get in melee range. Fire Arrow and the like are ranged attack, but also have ranges. I wish I could read Japanese so I could tell you exactly how far or close you need to be, but I can't. Good luck.
Also, I don't think there's a purpose to even fighting as her rn. I guess you could level her sexual resistance?