Which type of doctor should I see for a extreme case of athlets foot?

Your call. Podiatrist, dermatologist, internist....
Podiatrist. Your better off going to see a Podiatrist because they will not only help with the infection, but also give better advise on prevention. .
Dermatologist. A dermatologist will be able to take a small scraping of skin and immediately confirm the presence of fungal organisms. They also are aware of an entire array of treatment options if it is or is not athletes foot.