Oh my - so many cultural references.
The Boy Scouts (and Girl Scouts too) have badges that show you have learned something, mastered some skills, etc.
He is making a joke that there is a badge called "Witch Dating" and to earn it, he had to spend a certain amount of time with a witch. He hadn't earned it yet (it kept eluding him) so he had to keep trying by dating a witch.
Be aware that calling someone a douche bag is quite rude. It's just a vulgar way of saying someone behave in a very unpleasant and rude way. She is implying that he is so adept at acting unpleasant and rude that he should have earned three of those badges by now. (In fact, you can only earn a badge once.)
Student or Learner