Elite Guard #1: After dropping down the throat at the beginning, defeat the monsters and go upstairs. Check under the bridge (mind the poison) for a skull switch that opens up a room on the far (lower) right side. The Elite Guard is here.
Elite Guard #2: After moving through the passageway with the slowly dropping ceiling, turn left into the poisonous area. The Elite Guard is at the end.
Argent Cell: After moving through the passageway with the slowly dropping ceiling and defeating all creatures here in the poisonous area after hitting the skull switch, check out (facing the yellow door) the right side of the area for pickups and this Argent Cell.
Classic Map: In the area with the falling platform, ride it up and hit the switch. Return to the previous poisonous area and turn right to find the map. You can also find a Mega Health behind a secret wall inside the classic map.
Keenguy Collectible: In the large room with a demon horde and the purple Quad Damage powerup, defeat all demons and check out a newly opened niche by going upstairs and checking the corner of the area.
Silverguy Collectible: In the large room with several Lost Souls, Unwilling and explosive barrels, use the jump pad to reach higher ground and move from platform to platform to reach the corner platform with this collectible.