Set up a line of legionaries to the front, and another one slightly behind. Set all to fire at will. That is what I remember doing, it broke the initial cataphract charge and allowed a countercharge against the stationary enemy, and also some flanking with my own cavalry. Then I retreated to the hill to the front-right of the initial starting postition and used fire at will to thin out the remaining parthian horsearchers.

It's a tough nut, definitely, but certainly possible to finish with more than half the army intact.