An effective and easy strategy when fighting Bosses or other hard to kill creatures is to make them follow you until you reach an area transition door. Fight close to it and when your hit points are low, exit. The creature will not follow you through the door. Rest and save; then go back to fight. Usually the creature will not have regenerated. This trick can also work with normal doors depending on the monster; usually humanoid creatures will be able to open non-area transition doors.