Mission 16: The Plan
This key is found as soon as you get off the elevator on the 105th floor. Go to the right and around the corner you will see the gold key between two lost souls.
When on floor 106, go to the right and there will be two hallways. The first will have red/white opening in the floor. Hop to the center and enter the opening on the right to enter the stairwell. Go down the stairs to floor 104 to find the door.
When you reach the elevator on floor 154, Kat will tell you to go up and that you’ll “face five floors of hell.” Before entering the elevator, go to the right and around the corner will be the door.
Mission 17: Furnace of Souls
After the battle on the main platform with the Witch, hop across the blue grapple to the red and the next red point that reveals a blue one. Grapple across the blue one to the ledge and then grapple to the right to the large stone against the wall. You can then use the Osiris to cut through the blue door.
Once you pass the divinity statue, demon pull the red ledge to the right of it and it will pull out a platform that will allow you to jump over to the door on the right side of the area.
Mission 18: Demon’s Den
To get this key, enter the tunnel to the right of the large center door in the main chamber. Follow the cave until you come to a large hole in the floor. When you reach it, drop down to the left and then follow the ramp up to reach the key.
When you enter the cave directly to the left of the large center door in the main chamber, you can see the door on the far right before entering the generator room.