You'll probably need to do a course to get by flight-planning. That's been the way for everyone I've spoken to. Nav., and Perf. & Loading are a little like flight-planning in that they have process type questions (sums, maths, equations), but they're not overly difficult. You could most likely get by them by just doing self study. That goes for all the exams except flight-planning.
Go to the CASA web-site and download the required reading list for ATPL theory. The textbooks with an asterisk next to them denote the ones that questions are taken from (I believe...). Study those books, know them back to front, and you should be right.
And remember that from the date you sit your first exam (pass or fail) you only have 3 years to pass all 7, or you lose all credit for those already passed. A point a lot of people unfortunately aren't aware of.
Regards,
Turbofan