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Funnily enough, I found a second year Intro to Number Theory class really hard. I even found a fourth year algebraic geometry course easier. The two main things were that I found the proofs to be really unintuitive, so I struggled to learn them, and the assignment sheets were really hard (which everyone found to be the case).
Fourth year group theory had some difficult proofs and extremely hard assignment sheets (even the lecturer who was covering for the module thought they were hard), but the exam was easy. Also, fourth year Lie algebras had hard theory but relatively easy assignment sheets and a nice exam.