Here's what worked for me.
If you didn't make a map when you were down the basement as Grace, number the rooms 1-13 from left to right (the NW room being #1 and the SE room being #13).
Close the west doors in rooms 2, 6 and 10, but do it quickly because that sequence is timed. Now that you've closed off Von Glower's access to the outside, you must corner him in the furnace room (room #13). Close off the exits as follows:
Close the north door in room #12. Then move west and close the north door in room #11.
Now move west, north and east and close the north and west doors in that room. Now just follow a path to the furnace room and Von Glower will be forced to stay ahead of you.
From room #7, proceed to 8, 4, 3, 2, 6, 10, 11, 12. You should be in the room just north of the furnace room. If Von Glower is now in the furnace room, save your game.