How to counter Kassadin
Let us take a look on how to beat Kassadin. We will look at champions who are strong against Kassadin and who are weak against Kassadin. Use win rate, gold difference and creep score difference at minute 10 to find the best 
Mid champion who counters Kassadin. You can already win in champion select with the right counters for Kassadin. If you want to know how to counter Kassadin then you need to take into account the best picks vs Kassadin and the worst picks vs Kassadin. This Kassadin counter data is based on matches played by Pros on patch 15.2 and previous ones. It is important to see whether Pros counter picked Kassadin or whether they played as Kassadin.
Champions used by Pros to counter pick Kassadin
Looking at the matches from Pros (see table Pros counter picked Kassadin) we can see that they picked 
Viktor most often to counter pick Kassadin. The matchup Viktor vs Kassadin occured 19 times. Kassadin has 14 wins and 5 losses in this matchup. Learn how to counter pick Viktor to get another perspective on the data.
Lane outcome for Kassadin
When looking at Kassadin 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 Kassadin. The lane outcome of Kassadin consists of three outcomes: Win, Loss or Draw. Let's take a look of Kassadin 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 Kassadin.
Conclusion
Analyze the available data and learn how to beat Kassadin. To master Kassadin take also a look on Kassadin builds and runes for Kassadin. Good luck, Summoner!