MISSION 2
Classic Doom Map: At the start of the level, when going down one level, finish the Possessed and the Imps and take note of the walkway up above with some armor. Move to the end, climb up and pull the lever, then check out the nearby opened door for the first Classic Map.
Elite Guard #1: After finding the yellow keycard, go back to the security room and enter through the now accessible yellow door. Here, use the console to unlock two previously locked areas. The first is found in the nearby hallway (leading to the Gore Nest area) where a hatch can now be opened on the right side. An Elite Guard can be found on the left side of the walkway down the hatch.
Elite Guard #2: After getting the yellow keycard, head back up the elevator (from the Gore Nest area) and go past the Field Drone. The door on the right side of the hallway up ahead is now unlocked. In the area behind it, look for a ventilation shaft, crawl through and drop down. The Elite Guard can be found in this room.
Elite Guard 3: After going back inside the facility (after the part outside on Mars), move through the hallways and turn left. Check out the nearby hall, kill the Imp, climb up and find the Elite Guard.
Doomguy: After going outside and wasting all enemies in the area, walk beneath the area under the console and look for a crate with blood on it. Climb up, follow the ledge to the right, look around and climb up the higher ventilation shaft to reach the top of this area. In the far back you can find a ventilation shaft in which Doomguy can be found.
Bronzeguy: Right before the very end of the mission, in the circular chamber, head left up the crates and into the ventilation shaft. Bronzeguy can be found at the very end.