Elite Guard #1: When facing the blue forcefield, go down the left path and drop down the ledge on the right. Continue to the far end of the walkway to find this Elite Guard.
Elite Guard #2: From the area where the blue forcefield is, climb up the crates on the left and reach an area with a round building. Facing this round building, move up the walkway to the right of it. This Elite Guard can be found on the left end.
Elite Guard #3: Near the very end of the mission, after killing various demons with a Berserk powerup, go through the large doors and check the left side of the area to find this Elite Guard behind some crates.
Classic Map: At the beginning of the mission, after moving past the Rune Trial and up the steps to the left, check behind the barrels in the left corner to find the Classic Map Lever. Continue through the next hall to find the Classic Map.
Argent Cell: Facing the blue forcefield in the main area, head left and go through the door on the same level. Waste the Imps inside this room and move to the end to find this Argent Cell.
UACguy Collectible: After destroying the Argent Energy Filter in the garage, move through the garage (where a Rune Trial is also located) and drop down to the cargo room. A new stack of crates will have been placed on the left side; climb them to reach a small walkway high up. At the end lies UACguy.
Stealthguy Collectible: Head to the main area and go up the crates and to the left, where the second Gore Nest was. Climb the crates and go through the door on the upper walkway. First shoot a few regular Possessed out of the way, then step back a little and pull out your rocket launcher. Head upstairs when all demons have been wasted and turn right to find a Field Drone and a Data Log (on the desk). Go downstairs and walk to the end of the walkway, then climb up the ledges. Jump over to the other side. There's an ammo crate here. More important is that you can walk on the ventilation shaft on the edge (don't fall down). In the room that you can access this way (to the right, near the end of the vent shaft) is the Stealthguy collectible.