How do I get an S- or better on ARAM?I

I just finished playing a game on ARAM. I was playing as and one of my team mates was playing as . It was the best game I played as to date, but I unfortunately only got an A+. However got an S rank. I checked the advanced details to find out how she got an S so that I can do better next time, but it turned out that I had the higher total damage output to both champs and towers. I also had better cc and more kills AND total take downs. All she had over me was that she only died 3 times while I only died 4 times. Just how do I get that S- on ARAM.

Just so you know, I also managed to get S- or better on ARAM as , as well as before the rework. However I don't know how I managed to do it and I only got chests for (ran out of available chests). So yes, I am capable. I just don't know how.

P.S.- I got the S- on on my first time playing her.