How the hell do i get s- or s ratings as katarina

mastery lvl 6 seems so difficult to get. I got my first s- rating, then won a few others, but these matches had better scores than the one in which i got s-. S- rating game : Games that followed shortly: Dont ask me why im playing kat adc and jg, thats how desperate i am to get these ratings :>. The two games i was playing kat adc and one game was me playing kat jg.
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