The Math Olympiad topics indeed cover a wide range. In Math Challenge IV we cover many of the topics in Math Olympiad, and it would be a multi-year program to cover a significant portion of the materials.  We do not specifically target a certain contest such as USAMO or USAJMO, and neither do we focus on a certain difficulty level.  We aim to build up the knowledge points and problem-solving techniques as well as the solution writing skills.  Some easier questions can be a good use for practicing on the writing.  When we get in more depth in a topic, the questions we practice with will be quite challenging.