What do you mean by mastery in writing?
With my writing classes, I focus on mastery or the idea of learning something until you fully understand it. For example, some students may complete a paper in two drafts while others might need four, particularly in the first few assignments. Rather than allowing a student to simply turn in a "C" paper, I require continued improvement. Since I view writing as a skill rather than a subject, mastering it means that it will improve every academic area in which a student uses it including history, science, etc.