This battle took me ages as well, beat it playing on Medium 357 Kills 185 Men surviving

At the start of the battle, move all infantry into standard formation. All units fire at will.
Have the general charge the HA behind you, do not stop until it routs.
Have the other cav unit charge the one on the left.

Move the two frontal infantry units into the back, so your units form an 'U' Shape

By this time your infantry will be firing their javelins and routing the HA on the right, and the normal cav unit would've routed his target. Change his target to another HA.

Once the cav routs that unit, immediately charge into the rear of the enemy Cataphract, thats where he will be fighting and dying for the rest of the battle.

Once your general routs the rear unit, have him charge at the Cataphract general, he is stronger and he can definitely beat it.

Meanwhile the rest of the cataphracts will be suffering heavy casualties and the Eastern General will be charging, take him head on with your surviving soldiers. Once he's dead the battle is over, have your general mop up any surviving horse archers.