Real and Complex analysis not too bad. There are far more evil mathematical subjects than this. I always hated Group Theory, dunno why.

Continuum Mechanics and all the tensor notations and differential geometry material, that was by far the hardest course I ever took. Statistical Mechanics was like a walk in the park. Quantum Mechanics - not too bad actually until it migrates to the higher level involving particles, random matrix theory, group theory, operational calculus and gauge theories...

Heck even Classical mechanics is a bieaatch - I remember having to solve numerous problems involving variational calculus. I always thought before I started that Classical Mechanics revolved around F = dp/dt in simple inertial frames. Found out it is way more difficult than that LOL... I dont think I ever got above a B in this course where I could score an A in Quantum...

Not a Physicist, started off in Physics but swapped out for Electronics and Communication Engineering. Still had to learn most the physics classes because 1/5 of the degree was solid state stuff (the department had way to many experts in Semiconductors and they biased their under and post grad degrees heavily this way).