In Classic mode, you challenge multiple stages en route to a climactic final battle. The more gold you put onto the Fiend’s Scale, the harder the battles will be. In return, your rewards will get better and more abundant. The best way to earn gold fast is to win Classic battles on higher intensity.
In Classic mode, you can choose your next challenge on the route map. Looks like Master Hand is waiting at the end, but perhaps something might change if you raise the intensity…?!