Don’t know how I missed this one so far, but just re-watched Dario Argento’s Phenomena and it totally fits, though with a disappointing ending from a GU perspective. And with the added bonus of featuring Jennifer Connelly, just a year before she would go on to star in Labyrinth. (Her character is also named Jennifer.)

Jennifer is sent to a Swiss boarding school by her absentee movie star father (her mother having abandoned them earlier). One of the first things she learns is that there is a serial killer on the loose, stalking teenage girls. Jennifer sleepwalks, and she also has a psychic affinity with insects. Both of these end up getting her enmeshed in the mystery of the killer, with the help of a friendly local entomologist (and a lot of maggots – this is Argento after all). But they also get her labeled insane by the headmistress, who tries to send her to an asylum. Jennifer escapes and is able to start tracking the killer, but when one of her companions is murdered too, she tries desperately to get back home.

When the Adversary is revealed, there is sort of a one-on-one confrontation (actually, a few of them), but each time someone or something else ends up doing the dirty work, rather than the girl herself. She does however have a very creepy descent underground in the basement of the killer’s house.

(This being giallo horror, it was over-the-top, but the enjoyment of that was made more surreal for me by the fact that the copy I was streaming would periodically switch to the Italian dub version for a little bit, and then suddenly switch back to English.)