What is Xianwu's weakness?

Xianwu stepped out of the carriage upon reaching outside a rest house. He had changed his outer appearance since he felt his father could be in the same rest house. Chen Xi also stood beside her while fixating his fake mustache. 

Since they had taken a local carriage, it left after a while. 

"Shall we go in?" Chen Xin asked. 

Xianwu nodded at him and the two walked in. Booking a room for themselves, a servant in the resthouse showed them the room. After he left, both Xianwu and Chen Xin sat in bed to rest. "The Crown Prince and I will separately look for General Lee. That way, we can work efficiently and effectively," he stated. 

"Of course," Xianwu replied. "The carriage drive mentioned that this rest house is the most famous because of its sophisticated environment. I believe we can also find my father here," Xianwu asserted.