After trying and failing multiple times on level 10, I did it on level 1 "Sacrum". I did this after beating the campaign.
Set difficulty to "Ultra Hard". Make sure you have full ammo (play the trial levels to fill ammo on whichever gun you need ammo for). Go slow, and remember that you need all kills, all 5 secrets, and you cannot die. I used the pistol on single zombies, the auto shotgun or shredders on multiples or shotgun guys, the auto shotgun on the imps (if they were close, if they were far away I used the zoom shot on the chaos gun), and the rocket launcher on the flying fat things. Remember that there is a switch on a half wall about halfway through the level that you push to drop a wall revealing 2 shotgun guys. Also check your automap periodically to make sure you aren't leaving badguys behind, this level is quite linear.
This will also unlock
Here is the link to the collectable guide by ShadowEchelon91
And here is the part from THEIR GUIDE (I'm trying to give them full credit) that pertains to this level.
1/5 After you fall down into the area with the partially opened door and collect the nexus point, the next room has what looks like 3 beds or tables. The door next to them can be interacted with to reveal a secret.
2/5 At the red key elevator, climb on top and jump over the nearby glass wall.
3/5 Immediately after secret 2, head outside and jump on the rocks next to the left and then again into a small area with armor.
4/5 Right after collecting the Shredders, drop down and turn around. There is an intractable door here with a faint "interact" symbol on it. Open it to reveal a secret ore.
5/5 After collecting the red key, immediately go left to find a ramp. Follow to the end for secret.