The Final Attack is Chapter 31 of Tsubasa: Reservoir Chronicles.
Kyle strikes at Sakura with a dagger, but Syaoran appears just in time to save her and takes the hit instead. Suddenly, the walls begin to break; it's the water breaking into the castle. The device controlling the water reservoir had broken and so, the castle's walls won't hold much longer. Syaoran pleads the others to leave and take the children with them, though hesitatant at first, they leave. Fai assures them that they will escape, because Syaoran does whatever his mind is set to do.
Syaoran and Sakura rush towards an exit, but Sakura stops him, saying that there's another way out from a secret passage that she memorized.The walls crumble and the passage blocks. Kyle appears, asking them to give them the feather. Syaoran tells Sakura to flee, while he takes him down. She should be able to go back and take the original passage to the left. But Sakura holds onto him, saying she can't leave. At this point, the water level will keep on rising and they won't be able to escape.
Emeraude appears and helps Sakura showing them another passage. Sakura starts hitting the wall, showing Syaoran a secret door. Syaoran looks confused, so Sakura tells him it was Emeraude who told her. Syaoran gives the wall a strong kick and a new passage is revealed. Kyle is still chasing after them when the castle walls fall and break over him.
Outside, the people is worried waiting for them, since they haven't came out and the water current is getting stronger, they won't be able to get across the river.
Fai and Kurogane sigh, then Kurogane states: they are here. A splash of water comes out, and both Sakura and Syaoran appear.
Kurogane asks where the doctor is. He didn't come out, so that would mean that...
Afterwards, the current takes the whole castle down as Sakura sees Princess Emeraude next to the children. She thanks Sakura for having saved them, finally she gives her the feather back, pleads her to be careful since someone had been watching her.