Kai Syaoran needed to get the Grand Water Immortal's attention, and he had to do something that would make the immortal see him as a threat. He looked around, analyzed the chaos, and landed on a single possibility. He would use the limited power he was willing to show the world to irritate the Grand Immortal enough to want him dead. It was no different than a mosquito buzzing near a person's ear, and the consequences were clear.
Kai Syaoran laughed at the fact that even with all his power, he had resorted to these kinds of attacks. He was powerful enough to end this war with a single word, and yet this was not something he could do.