@WillLi sorry didn't reply sooner was reading and thinking and yeah you were probably right. I'm not denying that this is not the best arc, ranted in a little (a lot) fit of rage with little to no thinking, but I'll put a hold on my judgement sorry.
@henkingu She didn't know about the regenerative spell beforehand so she thought that the other objects were more useful and in an normal scenario they are, the blood mirror even the girls said that it was useless, but when the full picture of the situation was revealed she thought a plan with the weapons she had left.
@henkingu
None of them thought about using speed and mirror this way before. It's like Kazuya's mother with Kiriha - what mere obi can do? Turns out obi tsukumogami can do a lot more than just caring things around. This is great learning experience for them, and that's what this arc is about in the first place - training.
So how many chapters is it going to take to beat her now... You'd think this bitch has magic cores in her body... I don't see how the house didn't crush what was left of her body