I know it’s really hard! I had three ducks survive being attacked by a predator several weeks ago and had to do the twice a day dosing for a couple of weeks. One of mine stopped eating, too, and lost a lot of weight. It’s nerve wracking and heartbreaking.
What worked for me was to prepare the medication in the oral syringe and set it up in the bathroom where my ducks were in the bathtub hospital. (I lined the tub with lots of newspaper and just kept layering more on as it got dirty, then rolled it up and bagged it when I cleaned. The thicker layers of damp paper were easier to manage.) I picked up each duck I turn and wrapped her in a large towel, then set her on the counter and held her like a football in/under my non dominant arm/hand. I wrapped the towel high up around the front of her neck, gently, so that it helped keep her steadier and restricted the amount of moving around. I think it was less traumatic for them to be wrapped in the towel because it kept their wings and feet close to their body and they fought less.
When I had them positioned, I picked up the oral syringe with my dominant hand and placed it at the corner of their bill, right where the bill meets the feathers. There is a little space there that is a bit softer and liquid inserted there will go into their mouth without having to get their bill open. I gave only a couple of drops at a time and waited a few seconds before trying the next drops, to give my duck a chance to swallow. If I couldn’t hear noises that suggested she was swallowing, I gently tipped her bill up slightly to encourage the liquid to go down. The duck’s windpipe is the big hole you see in the center of the back of the mouth when they open their bill. The liquid needs to go down the crevasses on either side of the windpipe to get to their stomach. That is why I gave only a couple of drops at a time. I didn’t want any medicine to enter the windpipe. It often took five minutes of giving a couple drops at a time to get the whole dose in.
Because the meds were crushed powder suspended in the liquid, I had to keep shaking it and even so, it would get clogged up in the syringe near the end of the dose because all of the liquid would be squeezed out and the powder remained. I kept a cup of water on the counter so I could draw a little more water into the syringe to dissolve the remaining powder and give it to my duck. My vet told me to do that to make sure she got all of the meds.
The towel really helped me keep my ducks feeling safe and secure. Use an old, large one, since your duck’s claws will snag it. Had I been able to lower the lighting somewhat, I would have, just to lower the stress for both of us.
You will get better and better at this each time you do it. If you feel calm, your duck will feel calmer. After a couple of weeks, I felt like an old pro, but up front, I was really worried and stressed out.