I would say around 50 to all stats is when you can really start going through threatening with all characters fairly easily..... 
My stats are 24 Constitution, 24 Willpower, and 18 Cunning. I can roll many kits on Perilous, and all of them can do Threatening blindfolded.
All you really need is enough materials to craft the highest-tier armor; enough game knowledge to read the tells of enemies, prioritize threats, and keep yourself out of major archer or wraith firing range; and enough chests unlocked to get in the "sweet spot" to grab yourself some nice purple weapons to aid the effort.
If you doubt any of that, feel free to watch my now-daily streams I'm doing in my hunt to unlock a Superb Ring of Life-Drain. From the response I've received so far, I'm not quite as scrubby as I initially thought and promotions definitely are not a major threshold to something such a Threatening. I was actually keeping it on that difficulty because I didn't think I was Perilous-ready with some kits at some levels... but with some encouragement from the Twitch chat crowd (and alcohol, LOL), I bumped up to Perilous much quicker with what I felt were under-leveled or under-promoted kits and did just fine.