What is hardest to learn is different for everyone. It is mostly a process of learning, however, and not a problem of "doing." You can do anything, with enough practice.
For me, the usual drills that others find hard were easy. I loved mask clearing. I had no problems with ear clearing. ESAs and EBAs were fun.
But for me, the most difficult drills to learn were the ditch and replacement of the B/C with tank on the surface and underwater, at the pool. But after enough practice, these also became easy to do.
Everything is just a matter, first, of trial and error, and then a matter of practice.
It is not rocket science.