I thought the shelf was difficult, but not impossible. I scored well on the exam and did not feel that I put ridiculous amounts of time into studying. I read blueprints one time only and did the questions in blueprints, blueprints q and a, and blueprints clinical cases (about 385 questions between these 3 books). I did all this over the first 6-7 weeks of the clerkship. The last 5 weeks of the clerkship I spent doing pre-test, mksap 1 and 2, and reading sections in cecil's if I found myself consistently doing poorly on certain sections. Most importantly, I finished all the questions on the exam, but did not have a chance to go back and double check. This strategy worked for me as I scored in the 96th percentile.
IMO, the most important thing to do well is to work briskly and be efficient with your time. Do not dwell on questions that you do not know the answer to (guess and move on). Also, read the answer choices before reading the passage because for a few questions (probably less than 5), the question can be answered with one or two facts from the passage. For example (this specific one wasnt on the shelf but was in mksap...the shelf had similar scenarios), they may give you a big long history of someone with CHF. They will tell you all of their signs and symptoms (orthopnea, swelling, sob, cough, s3, jvd with hjr, pitting edema, and so on and so on) and all of their labs. But the question is what drug has been shown to decrease mortality in this pt. You would pick the ace inhibitor and move on. You do not need the entire h and p to answer this question. If you do not do the last 10 questions and guess on all of them, you may get 2 or 3 right. If someone else reads through all 10 and gets 7-8 right, the extra 6 correct answers would be a difference of about 30 percentile points. So work quick and make sure you do not dwell on questions that are ridiculously hard. There are about 10 experimental questions on their that arent even graded so if a question seems vague or poorly worded or beyond your level of education, it probably isnt even a graded question.