looks like you need more cs
+more wards more teamkill/assist contribution more damage less damage taken more tower kills more dragon kills.Overall just more skill
Probably more wards and vision wards too.
Cloud get S- with 12/13/15 vladimir but u can't get S with 15/0
The grades are determined by comparing against the "average" performance for that champ. So i guess Kog'maw is just really OP?