How hard is calculus?
Calculus is widely regarded as a very hard math class, and with good reason. The concepts take you far beyond the comfortable realms of algebra and geometry that you've explored in previous courses. Calculus asks you to think in ways that are more abstract, requiring more imagination.
What is the hardest topic in calculus?
I'd say the concept of zero, infinity, and the infinitesimal. Often they're thought of like numbers and this leads to calculations which don't make sense where, even if they work numerically, it's not actually correct since the process is wrong.