How the heck can I get an S grade on Caitlyn? What am I doing wrong?

So I am trying to reach mastery 7 on Caitlyn (just in time for the prestige skin lol), and I always had difficulty in earning S grades on her but I somehow managed to get to mastery 6. Right now, it feels nearly impossible to earn S grades on her even when I feel like I am doing relatively well. Take this match for example: Pros: -10 kills, 16 assists, no deaths -212 cs (most in the match) -Most damage dealt -Placed 15 wards, destroyed 5, and purchased 4 control wards -Earned most gold Cons: -??? And I only got an A+ on that game, I am so confused.
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