Getting an S on a damage character is nearly impossible in ARAM. Had a game where I did 88k damage (2nd highest was 50k), was 20/8/38, highest KP in the game, got a quadra kill, and had 2000 gold more than the next highest: riot gave me an A-. Other times I have played a tank like maokai and gone 4/5/20 and gotten an S+.
If the champion you got performs well in ARAM, generally, good luck getting an S if you win.
If the champion you got does not normally perform well in ARAM, good luck not getting an S if you win.
Grades are dependent on the averages for the champion in question when played in the queue in question.
Ive noticed that champions that tend to do better in aram are harder to get S with. Not overall well, just kill deaths and damage. like viegar, extreamly hard to get an S with even when I do 70k
