I'm looking to start drilling LR but don't know if I should buy all of the PDFs. I am looking at this link right now,
http://www.cambridgelsat.com/problem-se ... reasoning/
and I was wondering whether I should just buy the main concept or everything. It says that some types overlap, such as except questions and difficult questions, so is it necessary I drill those separately? Or will drilling just the main concepts, such as Strengthen and encounter the EXCEPT and difficult questions here, be sufficient?
Also, do I drill one type at a time and move on to the next when I have perfected it? Or do I consistently do each type so I don't lose what I learn?