Jiang Dina told them that Cao Wenxi was no match for her, but Gu Ning was quite strong and even kicked her away before. One of her ribs was broken at that time, and she didn't recover until a month later. In addition, she aimed to teach Gu Ning a lesson, so it would be the best if they could fight against Gu Ning.
If it was possible, she would like to see Gu Ning have one, no, two broken ribs too.
Because of that, the two ninjas felt much better.
It was a competition for fun, but they wanted to have the losers and winners and they didn't care about injuries, so they all used their full strength once the game started, leaving their opponents no chance to take any advantage.
Although Jiang Dina told them that Gu Ning was unusually strong, the two ninjas still didn't take it seriously. They simply wanted to finish the game as soon as possible, so the two of them accepted the challenge together.