Firstly, standard battle assumptions has her know her target, and she doesn't have the weakness of being cocky. Secondly, she knows that something is depleting her aura at a massive rate and pretty sure SBA has her know that it's the rat. Thirdly, SBA has her able to see the Stand, so she sees some Grimm looking monster controlled by the rat. Weiss is really smart, after all. Fourthly, speaking of Grimm, even if everything I said before in this paragraph is wrong, she'll probably assume the rat is one, given they're trying to kill each other. Weiss doesn't hold back against Grimm.
Weekly, I'm saving Weiss from melting with my debating skills. You owe me one ovo.