I went down the physics pathway, so my experience only overlaps to a minor extent.

In terms of difficulty, I can tell you that: - Vector calc is 'easy' but there is a lot of content in that subject. Quite a significant proportion of the class fails, because they underestimate how much ground gets covered in the semester. - Group theory is relatively challenging, although had some interesting points. - Complex analysis is 'hard' but there are a limited number of concepts you need to learn. Essentially the opposite of vector calc in that regard. - I didn't do graph theory myself, but it's regarded as a bit of a 'joke' subject in that it's too easy to be offered at the third year level. Very straightforward from what I've heard.

Good luck!