How do you get an S rating on Diana?

I've been trying to get S- or higher on Diana (for master 6) but i usually just end up with B and if I actually thought I did very well I get A- I've only gotten S- and higher on ashe and miss fortune but the match that happened was when I had 23 kills 0 deaths or some nonsense like that. Any advice? Is it based on CS? damage, wards? I don't understand this rating system.
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