From one who did 4Unit maths when I was at school (same syllabus) and then tutored it after school...
It can be quite difficult, mainly because the concepts are VERY new. There are no complex numbers anywhere else in the syllabus, so it took some time to get my head around that. Then Volumes using integration was "interesting" but I managed to sort that, too. Mechanics was the hardest topic because they maths teacher was not so hot on it and, although I was doing physics at the time (it that was harder then, than it is now with mostly maths involved) my physics teacher was not very good at explaining the maths behind the mechanics either. It was only through tutoring it that I finally got that working. Mechanics needed some heavy duty understanding of differentiation and integration.
The thing with 4Unit/MX2 is that it is not rote/mechanical learning like the other maths. You don't just learn a formula and method and then can do every question. There is a lot of getting the feel for maths and having the confidence to try stuff and work through to an answer. In MX1 and below you can generally see the answer and how to get there but MX2 you may not be able to do that and just have to fiddle until things fall into place.
So, MX2 is not impossible, it's just different and requires you to appreciate the elegance of mathematics (to coin a Bored Of Studies thread) rather than be able to just apply it.
For the gifted mathematician and anyone willing to put in the hard yakka, it is rewarding.
I was in a class of 3. I used to get 80+, then the next girl used to get 50+ and the last girl used to get 20+. Yet, those other two would never drop it because they enjoyed it so much AND did quite well in the HSC because it scaled AMAZINGLY well.
The most ironic thing is, we had our ###th school reunion just recently and I spoke to the girl who got 20+% and she had become a Maths teacher and was just starting to teach 4Unit MX2!
You need a dedicated and interested teacher to do MX2, and hopefully they can guide your mind towards the answer without being able to necessarily do it for you. BUT in the end the MX2 student needs to be able to guide themselves most of the way. That is the intention of the MX2 (and EngExt2) courses.
Hope that answers your question.