Ji Aga was said to have Mou Bu level strength.
By Sou'ou. Are you really implying that the opinion and experience of Sou'ou is equal to that of Legendary Great General Ouki of the Qin Six?
If Duke can, imagine Ou Ki.
Duke Hyou did not beat Gokei on his own merit. He needed two solutions that he had zero control over or even awareness of (Shin+Kyoukai dealing with chariots and Ouki coincidentally coming across his battlefield while going on holiday to visit Renpa) to force Gokei off of his hill to give him the chance to close the gap between himself and Gokei. Even then, the gap was only eventually closed by additional factors outside of his control such as Heki + Shin launching an attack from the hill they could only hold due to Ouki's arrival and Gokei's stubborn personality to refuse to give ground to an invasion force.
Despite all that external assistance and outside factors, Hyou's Army was toasted to the point that it was unable to achieve the campaign objective to capture the target Wei city despite the death of Gokei. Gokei achieved his campaign objective, Hyou failed his.
Hyou, a man that was offered a position amongst the Qin Six, was hammered so badly by Gokei that he needed the author to toss actual Qin Six Ouki to save his hide. Hara had to Ouki ex Machina to let Hyou have a chance of winning.
TL;DR it took two Great Generals to defeat Fire Dragon Gokei.