Secret: In the first open area, go up the stairs (after dropping down first). Upstairs, check the left side of the rocks and follow the rock plateaus back up to an armor, which counts as a secret.
Elite Guard #1: This guard is simply behind the yellow door.
Elite Guard #2: After going through the blue door you'll reach a large open area. After defeating all enemies, check out the cave in the far upper right side and walk through the top of the cave to find this Elite Guard.
Elite Guard #3: After going through the blue door you reach a large area with another door. After opening this door and defeating all demons in the next area, look for a green portal and go through. This Elite Guard can be found at the end of the cave's walkway.
Elite Guard #4: Near the very end of the mission you'll reach a fairly empty, large square with some Imps and Possessed fighting eachother. Look to the left side of the square to find this Elite Guard.
Classic Map: In the area where you first encounter a few Barons of Hell (to your dismay), check out the cave on the opposite of the bridge/door to find the lever for this map. Go to the bridge again and drop off on the right, then turn around to find the entrance to this Classic Map around here somewhere.
Astroguy Collectible: After reaching the area where platforms rise (which is in fact the beginning area), you can make your way to the far upper end. Use the skull, then go down and through the red gate that has opened up below. Clear out the Unwilling inside the cave. Before going through the portal, check out the area to the right to find the Astroguy collectible.
Redguy Collectible: After going through the blue door you reach a large area with a door. After opening this door you reach another fairly large area, with a portal, a haste powerup, a rune trial and so on. After defeating all demons, go up the stairs and stop halfway, then turn right to spot a rectangular shaped rock formation. Double Jump to the top to find the Redguy Collectible.