I’m going to assume that the physician has a normal med school education, and has miraculously landed in a position where he or she can practise in the past, and further that his or her accomplishments gradually allow greater scope, due to the patronage of the powerful.
Surgery: sterilization and clean operating rooms, as discussed above.
OB/GYN: Prenatal care (stop drinking), clean deliveries, obstetrical forceps and the end of childbed fever
Oncology: Cut early. Cut as far away from the lump as possible.
Radiology: Nothing.
Nuclear Medicine: Nothing
Psychiatry: It’s not demons - just disease. Get patients someplace clean and compassionate. Invent psychology until you have time to invent pharmacology.
Neurology: Same as psychiatry.
Internal Medicine: Mostly you’re gonna have to work on lifestyle, surgery and inventing pharmacology.
Community Medicine: Clean air / water / drainage / sewage / eradication of disease vectors. Invent socialized medicine, statistics and the scientific method while you’re at it.
Genetics: The entire field of genetics is there to be invented, and some efforts encouraging outcrossing would go a long way, not least among the royal families of europe.
Microbiology: If you can encourage the invention of the microscope, as well as the concept of disease being caused by microscopic animalcules, you’ll have done a lot. In the meantime, make people do things because you say so.
Diet: Good luck with this one. That scurvy thing may help our patron and his navy become strong and powerful though.