How to get S rank on supps like Soraka?

As the title says. I can't seem to get S rank on soraka, the highest I got is A+. This comes from having 30+ assists and dropping tonnes of wards and stealing those 1 or 2 kills trying to get rejuvenate. I feel like its extremely hard on supports who don't do much damage and don't need kills. Has anyone done it? If so, how?
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