after posting yesterday I tried Carrhae again and after a long struggle I was able to defeat the Parthians. I stopped the cataphract charge by using the four cohorts in front. I also sent the two cavalry units to attack the two units of horse archers in front of the cohorts. Once the cataphracts hit the legionaries I turned the cavalry around and used them to attack the cataphracts in the rear.

After a protracted fight, I had to use my remaining cavalry to hunt down all the horse archers while my remaining infantry formed a circle to ward off any cavalry charges

It was not easy