In the sim:
Instrument skills. A sim is a sim, its just a computer. Garbage in, garbage out.
Not knowing your profiles, callouts and flows dead cold before getting in. You will not be able to learn how to fly the manuevers if you are still trying to learn what to say or which switch to flip.

If you are good at those two things and arent a total moron at CRM then the sim is a piece of cake.

In ground:
If you lied about how many checkrides you failed or why you left an old job or forgot to put something on your application it will come back to you and you will get canned. Watched it happen to more than a couple pilots.

A good attitude, paying attention in class, and doing a little bit of studying will get you through the ground. Most people dont have to cram for hours every single night.

Do you have a class?