When we have to give a bird an oral medication or even water, we get in a seated position or stand near a counter. With the bird facing away from you, put the weight of the bird on one leg if seated or on the counter if standing, then put your arm around the bird like you're holding a football with her vent toward your back, her face looking in the same direction you are looking. Make sure they can't move their head backward or pull it downward; we use our chest as a backstop for the head when holding them this way to give meds/water.
Now, use the hand of that same arm to reach under the bird's head to open the beak using gentle pressure between the index finger and thumb at each side of the mouth. Once the bird opens its mouth, slip whatever you need to feed them into the back of the mouth with the other hand. You'll have to be fast as they won't hold their mouth open long if at all.
Sometimes we can get a bit of a handle on the lower beak and hold the mouth open a little longer, but care must be taken not to injure the bird if it's going to shake its head.
When delivering water/fluids to a bird, you'll really need to hold that mouth open for a second or two to get the end of the syringe/dropper in the right location, which would be to the back and side of the mouth. The trachea is the hole in the center and the place you do NOT want fluids to go.
If there is ever a question about your placement of the delivery tool for fluids, pull it out of the bird's mouth, and do the placement again. Be CERTAIN you have it in the correct location.
With a little practice, this becomes quite easy to do, even with large birds.
OK So your vet gave you the Panacur (Dewormer) and the Corid (Coccidiostat) to treat worms and Coccidiosis.
Are you getting the stimulation injection into the crop - through the layer of skin and through the crop wall? It's not getting injected into the breast muscle is it?
She's needing hydration. Syringe or tube the mixed Corid water into her taking care that she's not getting too much fluid and aspirates back from the crop.
Not sure how well the stimulation injection is going to help if she's full of grit. Not sure what will help with that since she initially presented with an impacted crop/digestive system/gizzard at the vet's.
Hopefully you can consult with her quickly and see what she recommends. It may come to needing to perform surgery to remove the grit. Since she's under vet care, it's best for the vet to do this and you not attempt to do this yourself.