Can anyone tell me the level of difficulty (approximately) of getting into QS world ranking top 150 theoretical physics graduate programs in Europe? ex. ETH Zurich, Durham University, University of Amsterdam, Uppsala University, etc.
HI, well if you are thinking about entering grad school in theoretical physics, perhaps the most important thing is to find a place where the specific research developed there has some interest for you. Theoretical physics is a broad field and in every university there are different departments with different research topics.
I will advice you first to have clear which specialization you would like to follow (for example, theoretical particle physics or condensed matter theory, cosmology.... every field has its own theorist working).
Secondly, to enter a top university is not easy in most of cases, but in grad school if you are thinking about applying this places, you need to have outstanding background, if that is the case, there are always grants you can apply for.
My final advice, its not about the institution you go to, but also about how much interested you are in the research developed there.