How to gain mastery

Without knowing what you need to score with a specifc champ it is ridiculous to achieve.

It should be based off of your highest stats or a little higher than medium of others.

I have played mid top and support with the same champ and have scored impressive scores such as 9/5/17 top or 12/3/17 mid or a 8/4/20 support with not even an s-. and its a support champ. what do I have to do play jungle with a support to try and ern an S score or kill steal. something needs to change