It is pretty confusing because the zombies just keep on coming.. If I remember correctly, you go through all the rooms, go up another level by stairs (thats the part Im not 100% on, its been a while since I did it).. go through some more rooms and then there is a half demolished brick wall at the end of a hallway, you go over that and there are some double doors straight across from it.. the bishop is in there with some more zombie friends.
Hope that helps
