In medical school, pharmacology was one of the most difficult classes.
Like Valerie and Shivani point out, everyone has different interests and ways of learning. Just work really hard and try to enjoy the process of learning. Do not be afraid to get tutors on those classes that seem too difficult. Sometimes, other people can rearrange information and make it easier to learn.
I am going to focus my answer of most difficult class IN MED school and not during my undergrad education. For me the answer is physiology. This course was difficult for me because it required knowledge and integrated concepts from all the other sciences (biochem, physics, chemistry) to understand how the human body functions. Reading out of a textbook or listening to lectures on: how kidneys filter, how the lungs expand and contract, how the heart beats with electrical activity--very difficult for me. I am a visual person so I need drawings and diagrams in videos to actually see these processes happen in an "animated" way to understand. As I changed my learning process it became easier but I realized that sometimes you have to ignore how everyone else learns and find a way that is suitable for YOU.
Hope this helps