If all the barons between you and the cyberdemon have been killed, move in front of another batch so the rockets hit them instead of you, and the fighting should continue until the cyberdemon is killed. While they are fighting, pick up the soul sphere in the room. Once the cyberdemon sees you, he will start firing, hitting the barons (who do not notice you until you decide to attack), who will fight back. First, go through the southern one to find a room with many barons of Hell and a cyberdemon. You appear at an octagonal room with eight nearly identical doors.
Sector numbers in boldface are secrets which count toward the end-of-level tally. Letters in italics refer to marked spots on the map.