I had to give my duck a lot of meds for a long period of time due to bumblefoot. I found that getting on my knees, with one leg on each side of her to contain her wings, making sure to create a triangle with the tips of my feet touching so she couldn't back out from my hold, then gently pull her head to my chest, holding between her bill and neck,(my chest would keep her from pulling back and my hand kept her from pushing down). and opening her beak with one hand and sliding the syringe down the side of her mouth, past the crucial airway in the center, then administering a little at a time, to give her time to swallow, (depending on how much med as you don't want it to pool and then spill over into her airway causing her to aspirate). This is the only safe way that I found to give liquid meds by myself. I made sure to never put any weight on her back.
I hope this might help. I apologize if it doesn't make sense as it's not easy to explain but I will try to clarify if needed. Wishing you all the best with your duck