I am afraid there is no better answer - the only way of getting good at solving the problems, is to solve as many as possible.
Actually they don't have to be limited to biochemistry - I would include a bunch of chemical and physics problems into the set. Probably even some some logical problems (like the Einstein's Riddle). Solving skills are not related to a particular branch of science, they are universal.
Sure you need a good foundation - but in most cases studying any undergraduate biochemistry textbook will do, unless the organizers suggest a particular title or particular subjects, then just follow their suggestions. There is no one "best" book, as a lot depends on individual learning techniques.