Openings are real important in any story, regardless of genre or even medium. The opening paragraph has 2 problems: The first line (which is fine) should be in a paragraph of its own, and the second line uses the word Towards twice.
Lots of description of the girl and her growth, but none at all of the demon(s)? That's no good. As the story stands, they don't really need to be demons at all. She could easily have explained that a wizard put the curse on her; other than that, I feel like you could replace the word demon with bandit and it would essentially be the same story. If the demons aren't important, they really shouldn't be in there.
On the other hand, I think there's a lot of potential in the premise. How about this:
Gabi was the best demonslayer in all the land, until her curse took her out of action. That was twenty years ago. Part of the curse is that she doesn't age (maybe it isn't a curse at all, but some kind of a side effect of killing a specific demon). One day she's walking in the woods and happens upon a novice demonslayer, who shows a lot of potential. Unfortunately, the newbie screws up her kill by making some novice mistake and Gabi has to come to the rescue. "Drat!" her boobs grow. Gabi is angry. The newbie begs and begs for Gabi to train her, but Gabi won't do it. Until she finally gives in. In exchange for sex.
By the end of the story Gabi is freed from the curse (but with huge tits) and is back in the game. Buffy meets Fast Eddie Felson. For me it would work better if the newbie was a nubile young girl. Then you can surprise the reader (or maybe not) by giving her growing tits about halfway through the story.
Anyway, that's my feedback. Like I said, there's lots of potential in the premise, and the above is only one possibility.