Incredulously means you do something in a way that shows you cannot believe it — you stare incredulously, you ask incredulously, you laugh incredulously. It is just incredulous with -ly stuck on: not the person, but the manner. Though actually, you almost always need an action next to it (said, looked, asked). People do not say “I am incredulously”; that would be “I am incredulous.” Same trap as the adjective: this is about not believing, not about something being amazing. It sounds a little written; in speech people often just use the face and “No way.”