Defensive itemization against mixed damage?

Hi guys I would like to know how to itemize defensively against champions going both magic damage and physical damage. For example, On Hit Teemo and Bruiser Fizz. My thought is that their magic damage from abilities does more early, so rush mr. Yet when I do that I am told to rush armor. What to do and why? And this si assuming I need to do so, so advice like "win lane so he can't get enough cs to get items", just no.
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