But if you do kill both of the two judges, all of the Imperial will run away, so I would suggest just nuking 1 of the judges with all your people first. Once this is done, just take down the next judge although, all your people will be getting battered quite heavily whilst you are nuking down the judges – so keep an eye out for your health.
By the way, once you get Ashe and you have made your way back to where you started, I would strongly advise healing and getting your MP back up full before you go into the room, because once your in, you have to fight Judge Ghis who is guarded with two Imperials. This fight is quite a tough one, but if you take down the two guards first, then have two of your party members attacking Ghis whilst the other heals you all, you should be able to survive.