What will best get out human body dirt/grease from a bathtub?

The reason bathroom scum is so bad to clean up is, dirt and oil becomes trapped UNDER soap films. Start with a razor blade and scrape off all you can, be CAREFUL if the tub is acrylic, you can DAMAGE the tub. Once you've scraped off all you can, then try your cleaners again. Dish soap and grease cutting agents like 409 or Fantastic will most likely work the best.