As the topic says, let's say that Duan Yu was Emperor of Dali at the time it was historically overrun by the Mongols. Or let's say that Duan Yu was invited to Ling Jiu Gong or something by Xu Zhu and Duan Yu walks through a secret Xiao Yao Pai time travel portal and is transported to Dali during the reign of Duan Zhixing.
Would Dali have been able to withstand the Mongols if they had Duan Yu on their side?
On one hand, the Mongol army is too large for one man to obtain military victory in a straightforward fashion against it. And even powerful martial artists such as WCY, HYS, GJ, ZBT, Yideng, and Huang Rong were unable to help their side obtain victory.
On the other hand, Duan Yu's martial arts were much greater than those of the aforementioned heroes, and the Mongols perhaps would been astonished at his ability to simply pass through largely any normal military force and kill leaders at will, as if he was a warrior sent by Heaven or a demon out of legend.