HOW TO GET A GRADE OF "S" SERIES?

Yesterday, i was playing and as i am mastery lvl 5 with him id want an S- or S or S+ with him for a token. I did my best and went 25/3/17 where total team kills were 52. I was hoping for a very good grade but riot gives me A+.....
here is the link: http://matchhistory.na.leagueoflegends.com/en/#match-details/NA1/2255650443/221781788?tab=overview i even got 1 PENTA AND 2 QUADRAS.

Pls suggest how to get a good grade.

Thanks.