I’ve spent way too much time trying to figure this out myself, and AO3's system isn’t always intuitive for a fandom-specific tag like that. The main thing is, 'Epithet Erased' is tagged as a fandom. So first, you need to go to the main 'Epithet Erased' fandom tag page. Once you're there, you’ll see the 'Filter' button. This is where it gets a bit fiddly.
In the 'Filter' form, look for the 'Additional Tags' field. That's the spot. Don't use 'Characters' for the epithets themselves—that's for character names like 'Giovanni Potage' or 'Sylvie'. The actual epithet words, like 'Dumb' or 'Soulless', are almost always treated as Additional Tags by the archive. So you’d type, for example, 'Dumb (Epithet)' in that box. Sometimes people just tag 'Dumb' without the parentheses, so you might need to try both. Hitting 'Sort and Filter' after that should narrow it down to fics specifically about that epithet's user.
It can be a real mixed bag, though, because tagging consistency is up to the writer. I’ve seen some fics tag the character and the epithet in the Additional Tags, and others only tag the character. If you’re looking for Molly, you’re in luck because she’s a central character tagged properly. For someone like Ramsey, who’s more of a supporting role, you might have to rely more on the Additional Tags search and just skim summaries. It’s not a perfect system, but adding the specific epithet tag usually cleans up the results a lot.