Back from the start, you can go inside where a Mancubus is trapped, open a secret wall (linedef #1917) and grab a backpack
Down in the metallic zone, use a blue border (linedef #3239) and enter.
In the secret room that you could see from outside, use a gray zone on a brown crate (linedef #3113 and #3114), enter a T-shaped corridor. From there, you can bypass the big elevator to the yellow key zone, or enter the room on the left of the elevetor.
Just on the left of the yellow key, jump down on the ledge with barons of hell and hell knights, shoot the switch and grab cells.
Go down in the large blood river, use the two switches (one per keycard), enter the teleporter (only once). You will activate crushers, and finally telefrag the cyberdemon and get a soulsphere.