Here is the thread from when my hen got hurt two years ago:
https://www.backyardchickens.com/forum/viewtopic.php?id=53714&p=1 I flushed the wound with betadine and the vet gave me an injectable called metacam to se as a painkiller.
Some helpful tips from the thread:
By jjthink- My understanding is that peroxide is not the thing to use. 9 parts water to one part betadine to cleanse wound is what an avian vet told me. Apply topical neosporin to the wounds.
You can get Rescue Remedy from the health food store to help with the incredible distress she must feel from the attack. A drop in her drinking water. Change at least daily.
Very concerned about the "bleeding heavily". That is the biggest problem at the moment. Keep her still and in calm environment with comfy temps and comfy bedding. You can set her up in a dog carrier type container if you have one. Use soft towels perhaps, not bedding that will stick to her bloody butt.
She may well need pain relief but you must wait until bleeding has safely stopped. When she is not bleeding, 1 baby aspirin (81 mg, NO SUBSTUTUES for the aspirin) crushed in one cup water. If she won;t drink on her own you can ever so slowly dribble the medicine water with an eyedropper along her beak - stop if she gets stressed.
If she has deep wound sthe topical neosporin may not be enough. She may need oral or injectable antibiotics. Others will likely come along to weigh in on that.
By dlhunicorn-
you need to flush the area with sterile saline daily (you can make your own by boiling 1/4 tsp of salt in a quart of wtaer...make fresh daily) ...squirt the saline solution over the wound...use triple antibiotic creme the first week generously on the area to keep it moist though if you possibly can, then I would suggest you get GRANULEX V ... you can often find this in the Horse section of your feed store or sometimes in the pet medication section for dogs and cats... here is an online source so you can see the details of the product and what it looks like:
http://www.bullwrinkle.com/ShoppingPages/granulex.htm