My last 2 years of high school, I slacked off big time and skipped classes all the time. I still managed to keep at least a B average. When I first started college, I also skipped a lot of classes and managed to keep A's and B's.
Calculus was the first class I took that I would consider hard. The first two times I took it, I skipped a lot and realized a couple weeks in that I was lost, so dropped the class. The third time, I went to every class and did the homework and suddenly I just "got it". After that, it seemed pretty easy.
I actually really enjoyed Organic Chem and found it to be pretty easy. It did require a good amount of studying and memorization though.
Physical Chemistry was much harder, extremely boring, and was the reason I switched majors.
Discrete Structures for Computer Science was the hardest math class I have ever taken. It's all so theoretical and is so vastly different than any other math class I had taken. No caculators required because there are basically no numbers involved. It's more just logic theory.
Oracle DBA was hard only because there were so many details to remember.