I'm so so sorry about your duck. That is awful...Are there any deep puncture wounds into the muscle?
I would recommend trimming away feathers from the wound as soon as possible. If you can get a hold of some Betadine surgical flush. Remove any debris and then flush the wound with the Betadine. After that, dab the wounds with some gauze until it is only moist instead of sopping. Then, try and keep the wound clean and covered for now since you definitely don't want flies getting at it. If you can put some gauze over the wound and then wrap around her body with vet wrap or provide a baby t-shirt (clean often when soiled) that should do the trick.
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You'll want to take her indoors or at least some place quiet and warm for her so she isn't bullied or in things that could infect the wound. I'd recommend giving her some vitamins and electrolytes for poultry in her water to help her perk up.
If the vet is an option, I would take her if she continues to refuse to eat, becomes very warm to the touch, or her wounds begin to smell or look very sickly over the next few days. She may need antibiotics.
Good luck