HSK Elementary/Intermediate is definitely not an easy test. Well, Chinese itself is objectively harder (at least for us Westerners) than many other languages, so getting at the so-called intermediate level is also harder...
As for advanced, I only took an official mock test some 2 years ago (it was like taking the real thing, they sent our test to the test center and we got official results, but we wouldn't get any certificates), so I might not remember everything correctly... apart from the test being harder on the whole, there are also some additional exercises not found in the Elementary/Intermediate level, one of them being "choose the (underlined) part of the sentence, which has mistake in it" (I heard that this exercise is included in the new revised Intermediate level as well) and some others, which I have already forgotten.