How to counter Senna
Let us take a look on how to beat Senna. We will look at champions who are strong against Senna and who are weak against Senna. Use win rate, gold difference and creep score difference at minute 10 to find the best 
Support champion who counters Senna. You can already win in champion select with the right counters for Senna. If you want to know how to counter Senna then you need to take into account the best picks vs Senna and the worst picks vs Senna. This Senna counter data is based on matches played by Pros on patch 14.9 and previous ones. It is important to see whether Pros counter picked Senna or whether they played as Senna.
Champions used by Pros to counter pick Senna
Looking at the matches from Pros (see table Pros counter picked Senna) we can see that they picked 
Nautilus most often to counter pick Senna. The matchup Nautilus vs Senna occured 42 times. Senna has 26 wins and 16 losses in this matchup. Learn how to counter pick Nautilus to get another perspective on the data.
Lane outcome for Senna
When looking at Senna 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 Senna. The lane outcome of Senna consists of three outcomes: Win, Loss or Draw. Let's take a look of Senna 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 Senna.
Conclusion
Analyze the available data and learn how to beat Senna. To master Senna take also a look on Senna builds and runes for Senna. Good luck, Summoner!