How do you get rid of really bad athletes foot rashes? Does sunlight (lots of it) kill the fungus?

Sunlight has no. Effect. When infected, you must treat it with at least a topical antifungal. Occasionally, an oral antifungal will be used if teh problem is severe enough. See your podiatrist or dermatologist, .
Couple of things. Exposure to sunlight does help. But you probably need to use and antifungal medication to help, as well as good hygiene.