To get that one secret you need to kill the Adult Arachnoid at 11:26 before entering the room. It's the other way on Serious as you need to kill the Arachnoid while you are in the room but not in the center of it (it's a weird set of detectors and triggers). You can also go around the ankh pool twice (before and after getting the statue) to get more kills, though I don't know if some of these spawners are Serious only.
To get that one secret you need to kill the Adult Arachnoid at 11:26 before entering the room. It's the other way on Serious as you need to kill the Arachnoid while you are in the room but not in the center of it (it's a weird set of detectors and triggers).
You can also go around the ankh pool twice (before and after getting the statue) to get more kills, though I don't know if some of these spawners are Serious only.
Thanks for the info on the missing secret. Interesting to know. Maybe I'll re-do this run someday if I feel like it.