Tactics against Comitatenses First Cohort?

[caspian]

I'm playing as WRE and after being driven off mainland Europe by both the Western Rebels and Eastern Empire, I'm rebuilt my finances and military in Africa and took on the Western Rebels in Spain.

As I entered the fray in Gaul, I noticed that most of the Roman armies I'm up against contain a lot of First Cohorts. My stacks of standard Comitatenses and Sarmatian horse can't even dent these monster armies. Most of the barbarians are wiped out and the only remaining powers are Roman.

I've tried engaging the front lines with Comitatenses and attacking the rear with Sarmatians but the First Cohors still held. They were exhausted, my men were fresh but they still wiped me out.

Any advice on how to deal with stacks half filled with First Cohorts?