We called the Marksman badge the toilet seat.
Yep, it's pretty hard to get an expert badge.
There are two phases to qualification. The first phase is the KD or known distance course. You'll be firing from 200, 300, and 500 yards. The second phase is similar to the Army qualification with pop-up targets, moving targets, and multiple targets. The KD course has a possible score of 250 with 220 points being an expert. The minimum score is 190. I'm not sure what the scores are on Table 2 but it has a possible score of 100.
If you fail to qualify you will be set back in training two weeks and make another try. If you fail the second time you will be DQ'd and sent home. You don't become a Marine if you can't qualify with a rifle or pass swim qualifications at a class 3 level.
As far as practice, there isn't much you can do to help you with the KD course. If you get someone who knows what they are doing or find good information on breathing and trigger squeeze, that might help. For Table 2, you will be firing from 25 yards. You could definitely get an advantage using any rifle to practice rapid and accurate shooting at close range. Learn to bring the weapon up rapidly, make sure it is level and on target. This will improve with practice. You need to be fast and accurate. You won't have time to think about aiming. Practice pointing and shooting until you can hit a chest size target repeatedly at 25 yards.
EDIT-- I just looked it up and 60 points is passing (not expert) on Table 2. The procedures are spelled out here: http://www.marines.mil/news/publications/Documents/MCO%203574.2K.pdf