There are a number of threads here that address tube feeding ducks, like this one: https://www.backyardchickens.com/t/725982/duck-force-feed-help. See the links embedded in the thread.
Tube feeding formulas generally seem to include a specially-formulated parenteral nutrition liquid like Ensure...
Sorry for your loss. :hugs How long has it been since your injured duck ate? What injuries did she suffer and what meds are you giving her?
It's good that she's drinking. Depending on how long it's been since the attack, she may be in shock, or pain may be impacting her appetite - more info...