Systems
I loved this subject.
Gavin Secombe (in class) followed by self study and A combination of Secombe and AFTs practice exams, it was easy. Having said that I come from a trade background.
The trick with this subject is time, the more the better. In most countries this is 3 Subjects (Aerody, Systems, BGT) hence don't treat it like other ATPL subjects and give it due diligence.
Get as many prac exams under your belt as you can, preferably from different sources (they all write questions for CASA). The way a question is written can sometimes make all the difference and if you're used to that line of questioning, your half way there.
Good Luck.
Baz...