Tips on getting an S grade on Tristana

Ok so recently I've been focusing on getting better at three champs - Tristana, Miss Fortune and Ashe. I've managed to get S grades on Ashe and MF but have failed on Tristana. The best I can get on her is always an A+. I remember once, I got the same KDA on Miss Fortune as Tristana, I got an S- on MF but an A+ on Tristana - why is that? So I woul just like to know any tips on getting an S+ with Tristana because I'm really struggling to do so with her. Thankies uwu
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