The npc will eventually use an exit that leads to a ledge outside. All along when the npc is seeking a way out it will first head towards the main exit. After you and the npc disposes of all of the mobs it will declare that the front entrance will be blocked then head back towards that other exit.
It would be very helpful if the profession one plays can mezz as the npc although tough will die due to the multiple adds. Also getting the mobs to focus their attention on you will allow the npc to do steady damage on the mob without taking any damage.
When the npc reaches the exit and assuming all the mobs are dead. Target the npc and a small conversation will ensue. The mob will explain something about her situation, bow, and you have completed the mission.