re: What's the hardest class you took in college.

I was a physics major. Junior year, you were required to take a year of electrodynamics, which was basically applying Maxwell's equations to all kinds of phenomena. Two years of calculus was a pre-requisite, and the "problems" (usually of the "derive an expression for" variety) involved wall-to-wall 3d vector calculus and partial differential equations in various coordinate systems.... A midterm exam consisted of one problem, for which you had an hour. If you were stumped, you were fricked. All you were allowed for exams was pencil and a blue book.