What do you focus on specifically?
From Everything I Needed to Know in Life I Learned from Jackson Electrodynamics
Undergraduate E&M is about solving the simple problems exactly.
Jackson E&M is about learning to approximate reliably. The entire
book, with few exceptions, is a mathematical discussion on finding way
to solve only 4 equations for different boundary conditions. Most of
the time, this requires series expansions and other approximation
As one comment implies, there's little chance of preparing for this course in the time frame you mention. While the following from the linked document may exaggerate a bit, it's not by much.
One Jackson problem takes an average of 1.5 weeks to finish. Most of
this time is often spent working on the first part of a multipart
problem. An approximate breakdown of the timeline of solving a
Jackson problem is:
a. Days 1-2: Arguing about what exactly the problem is asking, what
assumptions to make, why the problem can’t be done as stated, why
Mathematica cannot handle the integral, why Jackson probably didn’t do
any of these problems, why the intial 10 pages of algebra failed to
deliver the correct answer.
b. Day 3: Rechecking the 10 pages of algebra for a missing minus signs
and factors of 2.
c. Day 4: Starting the problem over the exact same way as before since
it is not clear where the algebra mistake came from.
d. Day 5: Discussing with the professor and realizing the problem is
not as easy/hard as previously thought and that 4 days were wasted
doing the problem the wrong way.
e. Day 6: Reworking the problem this new way: 13 pages of Algebra.
f. Day 7: Realizing this new way didn’t work either, and discussing
with professor why it was wrong. After getting an extension and
“knowing” the correct way to do the problem, swearing it will get done
g. Day 8: After working 15 pages of Algebra, you realize that a minus
sign was left out on page 2. Reworking all of it, you are off by a
factor of 2 from the expected answer. Going back and reworking, you
find the missing factor on page 3 of 16. The first part of a 3 part
problem is now done. No other homework or research was none today (or
the past 5 days).
h. Day 9: Part b is not as hard, but still takes 6 pages of Algebra.
i. Day 10: Part c takes 2 pages of algebra and a page of words trying
to answer a conceptual question that no one cares about at this point.
j. Day 11: Turn in the homework assignment, and reflect on how much was
learned and how horrible the problem was. It takes 24 pages total