Champ Masteries rely a bit too much on CS
I think it should count CS and kill/assists as 1 criteria, where the more kills/assists you get, the less CS you need, and the more CS you get the less kills/assists you need to get an S.
Otherwise we get games like this one:
I was recently working on my champ masteries and I just realized that you really have to farm up a lot to get an S and that's not always realistic depending on how teamfights are going and how teamfight-oriented the game is. And no, that Trist was not me, but I think that champion masteries are really encouraging more AFK-farming than it should.