It's not easy, but it's not extremely difficult. Achieving a 90+ ATAR requires determination and sustained effort throughout year 12. This will allow the student to rank sufficiently high relative to their cohort across their subjects (internally) as well as perform well in the HSC exams of their respective subjects (externally). Supporting this determination and sustained effort is a combination of subjects that the student either enjoys or has a particular interest in, which are factors that should be prioritised over other aspects, such as subject scaling. In terms of HSC marks, a student who achieves a 90+ ATAR will have likely have achieved a combination of band 5's/E3's and band 6's/E4's. With that being said, band 6's/E4's are of course favourable, as they maximise the student's chances of achieving a 90+ ATAR's, i.e. the more the band 6's/E4's the better. In terms of relevant data, about 17% of students received an ATAR of 90 or above. While this is not a huge percentage (one can clearly see that this is less than a quarter of those who completed the HSC for that year), it is not extremely low, especially when considering the number of students who completed their HSC in 2020, which was above 69000, meaning approximately 11730 students received a 90+ ATAR in 2020, therefore being a considerable number.
I hope this helps!