Experimental part is not necessarily related to advanced category as something difficult. However, due to operational challenges of buying different equipment, in many countries experimental section is non-existent at National Physics Competitions. Most likely this section will be useful to you only if you will be selected as a part of National team for IPhO/APhO

In Knowledge of fundamental physical constants chapter are listed the most common constants that are useful to memorize. It is not that difficult but can give you significant advantage for validating your experimental results

In Physical properties of common materials chapter are listed some properties that also could be useful for validation of your experimental data. Memorization of actual values is not required, but familiarization with certain ranges will be definitely helpful at Experimental Task 
Drawing plots chapter describes common pitfalls and expectations from participants about plotting their experimental data. It is recommended to print out a graph paper and practice with quick and accurate plotting based on the provided examples

Linearization chapter covers fundamentals of linear regression technique with useful applications for transforming variables for yielding linear relationship. In almost all IPhO/APhO experimental tasks you are expected to use linear regression for post-processing your experimental data