Is this a question that I should diagram? or will a negation test be sufficient?

Hi, I would lean against diagraming this, and would lean instead towards the negation test. One thing I like doing for necessary assumption questions like this is to treat them like a must be true. For necessary assumptions, we are asked what the author must agree with, and figuring out what must be true feels easier to me that finding the thing that if negates would undermine the argument, even though those are conceptually the same thing.

Ok cool. Treat it like a MBT - i'll use this next time!