I beat it for the first time ever the other day. I actually went into the battle with no intent to win, but to just show how tough the initial situation was. Typically, I managed to win... No real special tactics...just untestudo, and use the legionaries in back as a countercharge to the cataphracts. What might have helped was my cavalry. I used them to keep the cav archers off of my flanks by charging one unit back and forth to cause them to skirmish away. My infantry managed to route the cataphracts, and pincer moves with my cav and infantry allowed me to kill off a great number of their archers before my cav were destroyed. With only two units of cav archers left, I used the testudo with my remaining legionnaires on the edge of the map, and after the archers ran out of ammo, they routed and I won.

Overall lesson...as long as you aren't trying to win, a victory will almost be certain.