OK. Beat the Hag's lair. I didn't even stealth it. I did use Potions of Speed, but no Long Rest beforehand. No Magic Missiles either to kill the dupes. Just raw attack strength with a Cleric, Battlemaster, Ranger and Eldritch Knight with two familiars. Used 5e rules, though I will confess it gets pretty hard to determine if I'm hitting from behind or not or from raised terrain. I probably was. Still, I only used potions as Actions and tried to keep to the Hard Core 5e rules, and I could still beat her at Level 3. It can be done.
I will also confess, my first time through, though, failed utterly. It took me 2 tries.
