gosh how hard is it to get a S+???

Did you place down many wards? Because, as support, not warding ends up costing you points sometimes.

I am guilty of not warding much. I just got an S+ with Blitzcrank on my third or fourth game with him. I went 8/4/25(ish with the assists). I was good in the beginning with handing kills to ADC and others. I heard it's based largely off how others do with that character.

Although I'm not sure how it determines how well you do.

I think that it looks for hard engages and giving your team 3-4 kills they wouldn't have gotten in team fights in exchange for your life, because that's a lot of what I did.