That Moment when you get S grade on Karthus

how do u even get one? I've been trying to but the highest I've had is A+

pls teach meee... i just finished the game with karthus 8/1/10 and only got A rank.. but wen i played with yorick 14/5/7 i got an S+ rank... wtf..? o.O

i made my first penta with karthus and till now still the only one i made lol but the important thing are not only kills! i main karthus for alomst 6 month...but only had 1 s since lvl 5... i had play of 25/4/22 and stil an a+..and all that cuz my farm was bad ! NEVER FORGET TO FARMl