The Strongest Kiishim is the 19th chapter of Tsubasa: The Reservoir Chronicle.
The mysterious figure encountered by Syaoran, Fai and Kurogane is the "kiishimu." Previously, only Chu'nyan's mother has been able to contain her power, however, with the help of Sakura's feather, the Ryanban can also control her as well as lock up the castle.
Kurogane immediately confronts her, asking for the Ryanban's whereabouts. The Kiishimu replies that, while she would ordinarily be offended by such human insolence, she has not had visitors for a while, she will forgive his rudeness. Again, Syaoran asks if she knows where the Ryanban could be, and if he is even in the same castle. She replies that, unfortunately, she cannot answer his question nor could she let them pass forward.
Fai ruefully says that she's probably going to do something violent to prevent them from going anywhere, to which the Kiishimu assents, changing the landscape of the room with the help of her secret arts. She attacks Syaoran with what appears to be water, but instead, it melts his sleeve like an acid. While the appearance of the room is all due to the secret arts, the damage that is dealt is real.
As Syaoran and the other two try to evade her attacks, they also must avoid falling into the lake, which could injure them just as much as the acid spheres. They dodge and jump, and Fai realizes that if they're unarmed, they can't do anything but defend themselves. He asks Kuro to break off one of the poles, so at least they could have a fighting chance to break the spheres without touching them.
Fai tells Syaoran that they can't stay fighting with the Kiishimu the whole time -- that there is something that he needs to do. He tells Syaoran that the magic is weakest at the top, so with Kurogane's help, he goes to look for the Ryanban, crashing through the ceiling.
The Kiishimu notices that only two were left, but decides to make do with the two that are left. She breaks the spheres, sending a rain of acid down over Kurogane and Fai.