When a lonely woman get bitten by a mysterious werewolf one late night she needs to find the person who did it to find out how to break the spell before she herself turns into a monster.

I agree about cutting "mysterious": it means nothing.
I would cut "one late night" too because it's not relevant to the plot.
I would cut "the person", in a logline the shorter the better.

.I would take out mysterious and maybe add a new twist like: She has a silver bullet bomb implanted in her chest and if she doesn't break the curse the first time she changes, she dies