Larian is the DM. It's their responsibility to balance it. They can limit it in various ways. Have mechanics that evaluate character level and dumb down hag stats based on character level, for example. Or allow players to recruit masked adventurers to aid them in the fight, but only if game assesses the players has used 50% of spell slots or are not fully level synced or hp is low. So restrict Long Rest, but offer other solutions.
So instead of killing off the masked adventurers, if you reduce to 0 HP or whatever, they are just defeated, not killed. You can remove their masks and gain their help in the upcoming fight.
Stuff like this can be fairly easily done.
