I would take the legions from the iberian frontier for they were mostly needed in the front,as well as two more,the one from Armenia and one from Syria.Half the praetorian guard to encourage the other legionaries,and a newly-formed Roman legion,with roman citizens,trained like old days. I would use the light cavalry you mentioned to counter the Parthians' horsemen and hire some barbarian heavy cavalry to chop the lightly armed parthian infantry..For elephants i would use archers taken from the auxiliaries of the frontier. Well, with such an invasion and some good,loyial generals, Parthia would be Roman!