Instruments isn't too bad and I had a very good tutor for the brush-up course. Gen Nav is time-pressured in the exam so you need to be on best and intimate terms with your whizz-wheel and there were aspects which I found hard. Met similarly but again, the tutor for the brush-up course did well to get across some of the principles!!!
I also found AGK difficult because the subject is vast and I don't have a mechanical leaning! And neither have I ever flown a fixed wing aircraft!!
Different strokes for different folks - if I went on % marks, then the hardest would have been Ops Procedures!
Cheers
Whirls