The experience with the galactic beast had helped Yao Si with her ability's control. She could now vaguely sense its trajectory. As for the reasons her water ability kept targeting Huo Qi's flame...
Mu Xuan's speculated that since her ability was passive and could only be activated after being triggered by someone else's ability, the moment it was released, it would instinctively target the one it was the opposite of. After all, once she extinguished Huo Qi's fire, her water ability disappeared as well.
The solution wasn't complex, she just had to master the trajectory. But it wasn't a skill that could be mastered easily as her ability had a mind of its own. Who knew when she would end up tossing a planet again...