The level of difficulty you see on the test will be dependent upon your actual ability level on the test. So whether you should study easy, medium or hard questions depends on how you're doing right now.

Generally, you can learn a TON from medium-level questions that you can then apply to hard questions that have similar set-ups or approaches - so don't just study the hard ones. The other thing to remember is that, as you get better and gain the ability to answer harder questions, you also have to get more efficient at doing the problems you already know how to do b/c, as you get better, you will get a higher mix of harder questions... so you will need to pick up speed on the ones that are easier for you.