How to counter Sejuani
Let us take a look on how to beat Sejuani. We will look at champions who are strong against Sejuani and who are weak against Sejuani. Use win rate, gold difference and creep score difference at minute 10 to find the best 
Jungle champion who counters Sejuani. You can already win in champion select with the right counters for Sejuani. If you want to know how to counter Sejuani then you need to take into account the best picks vs Sejuani and the worst picks vs Sejuani. This Sejuani 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 Sejuani or whether they played as Sejuani.
Champions used by Pros to counter pick Sejuani
Looking at the matches from Pros (see table Pros counter picked Sejuani) we can see that they picked 
Lee Sin most often to counter pick Sejuani. The matchup Lee Sin vs Sejuani occured 21 times. Sejuani has 12 wins and 9 losses in this matchup. Learn how to counter pick Lee Sin to get another perspective on the data.
Lane outcome for Sejuani
When looking at Sejuani 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 Sejuani. The lane outcome of Sejuani consists of three outcomes: Win, Loss or Draw. Let's take a look of Sejuani 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 Sejuani.
Conclusion
Analyze the available data and learn how to beat Sejuani. To master Sejuani take also a look on Sejuani builds and runes for Sejuani. Good luck, Summoner!