Can someone help me with my getting an S?

So today i played Zoe mid and got a KDA of 13/0/9 with 12 vision score and highest magic damage on the team. I got 108 CS in a 34-minute game. I received an A- and it kind of tilted me. I just wanted to ask if anybody with an M6 or M7 (on any champion) can tell me how they managed to get it. It was my best Zoe game in a long time and the A- made me a bit angry. Advise would be appreciated {{sticker:sg-soraka}}
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