Transmigrated as a Castoff: I Become an Anti-Mongol Commander with Flintlocks

Who could have seen this coming? I transmigrate to the Southern Song as a minor official, and the Mongols are already at the gates! I strike back with volleys of matchlock guns and mother-and-child thunder bombs, launch a night raid on their supply lines, and burn their granaries. I hold my own against an army of a hundred thousand! Xuyi is secure. Next stop: the northern campaign against the Mongols!