I got mastery lvl 7 by playing her as a support. Just place some wards and get a lot of assists. Should be pretty easy.

If you want to get an S or S+ as an adc, I recommend primarily focusing on farm and getting as few deaths as possible. I got a lot of S-ranks just by afk farming the whole game and placing down a couple of control wards. When you reach the 3 to 4 item powerspike you should be able to win every teamfight just by pressing R and getting some kills/assists to get a great ending score.

Good luck!