Interested in doing Master's in ETH, how hard is it? I have been researching universities and I'm really interested in pursuing Master's at ETH Zurich in Bioengineering. I noticed a plethora of information in their website and it differed from the information found in the websites of U.S. and Canadian universities (e.g. they didn't mention obtaining a supervisor before applying to the program). 

I have a bachelor's degree in Biology from Canada (with a somehow low GPA). My question is, how is it hard to get into ETH? I realize it's competitive, but what is the general portfolio of attractive candidates?
I noticed they required GRE, is it absolutely necessary for students who studied in North America?
How hard is it to get into ETH? 


The Times Higher Education World University rankings lists ETH Zurich as 14th in the world for life sciences. This is below universities such as Yale (7th) and Johns Hopkins (11th) but above universities such as UCLA (18th) and University of Toronto (22nd). This ranking implies that the admissions process will be very competitive, as such a well-regarded university will only attract the best candidates. 


  What is the general portfolio of attractive candidates?


An ideal candidate will likely have experience of performing undergraduate research, a good degree classification or GPA and very strong reference letters. Additionally, they will have clear motivations for wanting to pursue the degree, as well as some idea of their own particular research interests. These would be included in a statement of purpose, personal statement or covering letter. 


  I noticed they required GRE, is it absolutely necessary for students who studied in North America?


If they state on their website that they require the GRE, then this suggests to me that this is definitely a necessary requirement. If you're still unsure, contact the admissions office of the department you'll be applying to.


  (...) they didn't mention obtaining a supervisor before applying to the program.


This is not uncommon, particularly for a partially taught Master's course, where in the first semester or year you will have lecturers and coursework, allowing you to develop your interests and get to know potential supervisors, before choosing a research project and supervisor for the second semester or year.

This question asks about maximising one's chances of admission to US PhD programs and I think much of what is suggested in this answer is applicable to your situation too. 

Lastly, what have you got to lose by applying? If you don't try, you'll never know.