5 hours?! That's insane. Did you only focus on that boss, or did you come back later and attempt other bosses?
She's been focusing on one boss/stage at a time. Hilda Berg is just on another level compared to the other world 1 bosses. It doesn't actually LOOK that hard, until you try. There's an amazing, amazing amount of spacial awareness required. Bullet/laugh patterns can suck, Gemini's attack pattern can suck and coincide with terrible RNG from other enemy appearances, Sagittarius' homing stars are absolutely the worst (half the strategy is not letting them come after you to begin with), and then there's the final phase. It has a lot of HP, and dodging the stars properly (or trying to use them for missing parries) at the same time as being fully aware of the incoming UFO patterns and the right timing to trigger them is just monstrous.
Again, it doesn't LOOK that bad if you watch someone do it. The whole time Nephy was playing I kept trying to give her advice because I felt she was missing some pretty easy strategies. But then I took a crack at it. Took me about half an hour to (somewhat) consistently make it through the first couple transformations without getting hit, and only a few times did I ever get to the final phase with full HP in the hour that I played. The problem isn't only spacial awareness of enemies. It's awareness of yourself too. What weapon are you firing, how much special meter do you have, how many parries have you done so far, where do you want to be positioned for various transitions. And the one that surprised me the most was just making my hands do what I wanted. I've always been an adept gamer with really good reflexes, but trying to inflict maximum DPS with constant weapon switching, at the same time as managing special weapon use, going tiny for dodges, and parrying… let's just say there were many, MANY times where I just pressed the wrong thing or knew what I wanted to do but just completely choked the inputs.