I'm fairly sure her deaths in IN are actually of her own doing and not the protagonists. In ULiL, she has to use her "set fire on herself" attack to heal the damage that using her other abilities causes her (which I think is a mechanic that represents her reckless abandon in using her abilities more than a "using lifeforce to fuel spells" thing, to be honest; it just seems to fit the character more imo). Thus, I think it stands to reason that she was doing the exact same thing back in IN. Her attacks, for one reason or another, hurt her own body, even when it's just danmaku, so from time to time (between spellcards, for example) she just blows herself up so that she can continue fighting with an undamaged body. The fact that she starts having trouble doing this near the end of her boss fight could mean it's a painful/tiring thing to do.