well I have passed all 13 now with the exception of performance.
Of the ones you are taking, from hardest first the order would be, in my eyes.
INS
SYSTEMS
RNAV
OPS
HPS
Instruments is not HARD it is just another subject with a very wide band of content. my advice
Know your basic pressure instruments inside out, backwards,upside down etc.. usually many questions on blockages and leaks etc..
in my exam there was NO compass working out just the basic acc west what happens etc..
the other major part of the exam was autoflight and auto pilot.. these are quite heavily examined for some reason, so know the autopilot approach sequence, speeds and heights etc..
do the feedback, brush up on the notes but more than anythig just be confident and do your best..
as for the others
systems,rnav etc.
just know the stuff and you'll be fine
GoodLuck
Spitty