Fortunately, I have the folklore to fall back on.
Because the hag is going to be the great peril at the height of the story. Perhaps I could pitch my heroine's bildungsroman such that she's the right level to face a hag when it comes to completion. But it is also the end of the campaign centered on scouring out the magical blights and perils of a range of hills. The hag needs to be a fitting climax to a whole campaign.
Not a coven of hags. The problem with a group as the final conflict is that they get whittled down and so less dangerous -- the exact opposite of what you need.
Also, letting the heroine advance in level means she has more revelations of her mysterious backstory to unnerve people. (I think she's going to discover she has a connection to the hag some time before the climax.)
