In her heightened state she is very aware of what Alice is doing as well as what the spider is doing and so the moment Alice speeds up so does she.

She is light enough to be able to run across the branches easily and now she knows to avoid them, the webs aren't too hard not to fall into.

So she reaches the spider just after Alice, she does have smaller legs after all, and drives her dagger towards it. The spider squeals, lashing out at Alice with a leg, turning with pincers towards B.

B blocked as best as she could as the pincers snapped at her.