Elite Guard #1: After the Gore Nest area, go through a hall, kill an Imp and climb up. Turn around and move through the secret tunnel, pulling the metal grid out of the way at the end to reveal this Elite Guard.
Elite Guard #2: After the big demon fight (there's a Quad Powerup in the room), go to the back of the area. Instead of going through one of the two doors on the left or right side, go through the middle gate, which you can open by using the panel next to it. This reveals the Elite Guard.
Elite Guard #3: After moving through the bloody 'sacred' area, jump down the hole in the circular chamber before dropping down the elevator. The Elite Guard is in here.
Elite Guard #4: After deactivating the portal, go to the adjacent room and turn to the right. In this (upper) right room, look for a hole in the wall and go through to reach this Elite Guard.
Eliteguy Collectible: After opening up the gate to reach an area with Pinkies and artefacts on both sides, look behind the first or last artefact on the left side to reach a hidden pathway. The Eliteguy collectible can be found at the end.
Toxicguy Collectible: In the circular room with the Helix Stone, turn right and check, near the red panel for a deviant red light on the wall and interact with it to reveal this collectible.
Classic Map: In the unholy, bloody section, move to the circular chamber where you can call an elevator. Before calling the elevator, search, on the side of the steps, near candles and such, for a lever. The Classic Map opens up in the pit.
Argent Cell: This one is easily found in Olivia's room, just beyond the large circular chamber with the Helix Stone.