Janna how to S?

Honestly time doesn't seem to factor in when it comes to rank - if the game drags out you naturally have higher values in each field (you can also do less good and get an s rank at that point). There have been many occasions where people preform so well so that the opposing team surrenders @ ~20 and get rather confusingly low ranks (That's why i'm guessing it doesn't correlate time against actual stat values).

I have mastery level 6 on Janna and from my experience the best way to get an S on Janna is to get a kill with her, even one kill with her is enough to bring your grade up to an S+(depends on how many times you died too, so try to keep the deaths to a 3). The best way to get a kill with Janna is just ks a low health enemy with your W, which does a surprising amount of damage.