The 70-466 exam is the hardest exam for many people because your skills in MDX and DAX are tested.
MDX is a very complex multidimensional query language. There are few experts on that language. DAX tries to be easier and it tries to be similar to Excel, but when the query is complex DAX can be very tricky.
Optimizing the performance is also a really hard task because it can be a problem in hardware, design or just the query. There are many aspects to verify to improve it.
The easiest section is Reporting Services. It is easy to learn and configure.
Another topic is Tabular Databases. Microsoft is now offering a new Database model alternative to the traditional multidimensional. Tabular uses DAX, but the topic in general is not so hard to understand and learn.
There is no Microsoft Training Kit book for this exam and the MVA course is not enough to pass the exam.
However, there is a nice free exam to practice here:
There is also a paid version of the simulator exam that includes 150 questions, answers including useful references for the exam:
You need to press add to cart to purchase the simulator exam.