How to counter Cassiopeia
Let us take a look on how to beat Cassiopeia. We will look at champions who are strong against Cassiopeia and who are weak against Cassiopeia. Use win rate, gold difference and creep score difference at minute 10 to find the best 
Mid champion who counters Cassiopeia. You can already win in champion select with the right counters for Cassiopeia. If you want to know how to counter Cassiopeia then you need to take into account the best picks vs Cassiopeia and the worst picks vs Cassiopeia. This Cassiopeia counter data is based on matches played by Pros on patch 13.24 and previous ones. It is important to see whether Pros counter picked Cassiopeia or whether they played as Cassiopeia.
Champions used by Pros to counter pick Cassiopeia
Looking at the matches from Pros (see table Pros counter picked Cassiopeia) we can see that they picked 
Sylas most often to counter pick Cassiopeia. The matchup Sylas vs Cassiopeia occured 12 times. Cassiopeia has 8 wins and 4 losses in this matchup. Learn how to counter pick Sylas to get another perspective on the data.
Lane outcome for Cassiopeia
When looking at Cassiopeia counters you will notice that win rates alone are not conclusional. Therefore we provide an overview (see table Lane Outcome) where you can see the lane outcome of Pros who played Cassiopeia. The lane outcome of Cassiopeia consists of three outcomes: Win, Loss or Draw. Let's take a look of Cassiopeia lane and match outcome for specific matchups. Considering the lane outcome and the match outcome of a specific matchup, might enable you to find the right counter for Cassiopeia.
Conclusion
Analyze the available data and learn how to beat Cassiopeia. To master Cassiopeia take also a look on Cassiopeia builds and runes for Cassiopeia. Good luck, Summoner!