What does it really take to get over a s- rank?

So i have been trying to get mastery 7 with my main, nasus, i just played a game where i had 8 kills, 0 deaths and 7 assists, took 2 and helped with another tower, took a dragon, and farmed 120 (was a 20 min long game) and i warded as often as i could and still only got s-? any idea how to get higher?