I am not sharing any of my code because Project Euler asks you not to.

The highest difficulty problem I have done was number 84 (find the 3 squares that would be landed on most often when playing Monopoly with two 4-sided dice) which was ranked 35%. I used a Monte Carlo simulation that is discussed in the YouTube video "The Mathematics of Winning Monopoly" (https://www.youtube.com/watch?v=ubQXz5RBBtU).

I have also done 16 Project Euler Problems in C.

I also like the Bebras and Oxford Computing challenges and the 100 Programming challenges on GitHub (links below).