Possibly an alternative:

The problem is that there are two Parthian armies with two generals. While the kataphracts are devastating, so are the horse archers(gold level weapon upgrades.. damn it!). When you think of it the losses from the horse archers seem to be actually larger than the losses sustained from kataphracts.

I've noticed there is a very steep but small hill behind your lines, and an enemy general is stalking out there. At the beginning of the battle, as quickly as possible, send your general's cavalry and Roman cavalries to the hill to kill the enemy general(this guy's the leader of the horse archers) as fast as you can.

At the same time, sacrifice two~three infantry cohorts to stop the initial kataphract charge and buy some time, while the rest five~six cohorts run ASAP to the hilltop.

I almost won the battle on medium difficulty - I'm pretty sure it was by luck(but you need it in a situation like this! :) ). I managed to kill the general of the horse archers very quickly with minimal losses. Subsequently, the horse archers supported the kataphracts and charged with them to my infantry, but by some miracle, the 5 units of cohorts managed to hold the lines at the hilltop.

Incredibly, all of the enemy cavalry routed after a short but brutal fight. I've also sustained very heavy losses, and only about 30 infantry remained(total of all the five cohorts) plus 12 bodyguard cavalry. The Roman cavalry was all dead.

The only enemy unit left was the enemy general of the kataphracts, and he killed my general which was the end of it all - but this was as close as I came to winning the fight. I've tried different and same tactics many times, but never came that close to winning.