I actually have a green hag as a main villain right now...she is posing as a good religious figure in exile. I don't plan to make it easy. First, she will have minions. Secondly, she is hard to track! If she goes invisible and runs, the party better have a good plan to catch her! A big greatsword does little to someone who is no longer there. I don't think they are all that tough frankly. In fact, if they have a party ringing them, the hag has already lost. Mine is doing damage through trickery and engineering problems from afar. If the party somehow gets up on her, they will dump three spells right away and likely stop her in her tracks. I don't think green hags are particularly imposing combatants.