I know exactly how to do the mansion, where all the hidden crests are. Even when I take the time to kill EVERY mob, I still consistently run into multiple doors where I don't have the crests. This despite getting every crest available.
When I say multiple doors, I mean that I get every single crest up to a given door, am missing a crest so I use a skeleton key, and then later on I run into another door where, despite having gotten every available crest, I'm still short a crest. Or multiple crests!
This is a very frustrating design.
EDIT: As an example, on my latest run I was at a 4-crest door that required a snake, octopus, wolf and scorpion. Here are the crests that I had:
So despite have 21 crests on me I was short 2 crests. And I'd already used my skeleton key on a previous door thanks to a similar situation.
The streaky nature of the RNG makes this an untenable design, IMO.