Abdul Hadi Pasha
Banned
Find it difficult to conceive of Roman armies advancing on Mongol territory, so would think it would have to be Mongol invasion of Roman Europe.
Roman generals lured out into plains would get the Carrhae treatment. Roman generals who kept their wits would keep the terrain to nullify Mongol calvary tactics and keep their discipline (lots of testudo). If they're smart, they'll bring along plenty of auxiliary cavalry and make liberal use of greek fire to frighten the horses. Mongols ever go against elephant cavalry?
Mongol scorched earth tactics would be less effective since Rome would be used them and I doubt the Romans would succumb as easily to Mongol siege tactics. Assuming we can somehow compensate for the time/technology development.
If we want Europeans invading Mongol territory, maybe make it Alexander/Macedonians with their phalanx and sarissa
Romans facing the Mongols would be fighting 1,000 years later, and would have had experience with a very large number of similar nomadic irruptions, including the Huns, Avars, etc. This would have caused their military organization to change.
Which is what happened in OTL. We call it the "Byzantine Army" (everyone forgets the Byzantines
Romanos Diogenes took on the Seljuk army with a degraded version of this force, and it was a close battle. If it had been the army during Basil II's reign, the Seljuks, and by extension the Mongols, would have been toast.