My group are all Wargamers, so I would just let them play the "good guys" and I would take on the Barons forces. The hex map in Ruins of Warsaw is a little small so I would either make an enlargement or make something up on hexpaper with larger hexes
I would also convert the unit stats to the "Last Battle" system (make each ROW unit into two half squads for LB).......or just play a small part of the battle using th PC,s and give them fragmentary reports how the rest of the battle is going.