Dont let your teen ignore the parts of their subjects they find hard

Because these are the bits they should be spending most of their time studying. For the stuff they find easier of course your teen should spend a bit of time polishing what they know to make for an A+++ answer, but it shouldnt be the main focus of their attention.
Your teen will come out with much better results overall if they identify what they struggle with and spent some quality time sorting it out.