More space in the coop and run(or cull the older gals as intended to cut down on stocking rate).... and kill the weaker bird along with them. When the whole flock attacks one bird there usually is a good reason for it...that's an instinct to protect the flock from either disease or a barren member. I wouldn't start all these complicated separations, nursing in the house, etc. as they just make life hard on you, on the isolated animal and on the flock. Flock management, if done correctly, takes some tough decisions and keeping an eye out for long term results, not just short term solutions.