How to counter Amumu
Let us take a look on how to beat Amumu. We will look at champions who are strong against Amumu and who are weak against Amumu. Use win rate, gold difference and creep score difference at minute 10 to find the best 
Support champion who counters Amumu. You can already win in champion select with the right counters for Amumu. If you want to know how to counter Amumu then you need to take into account the best picks vs Amumu and the worst picks vs Amumu. This Amumu 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 Amumu or whether they played as Amumu.
Champions used by Pros to counter pick Amumu
Looking at the matches from Pros (see table Pros counter picked Amumu) we can see that they picked 
Senna most often to counter pick Amumu. The matchup Senna vs Amumu occured 3 times. Amumu has 1 wins and 2 losses in this matchup. Learn how to counter pick Senna to get another perspective on the data.
Lane outcome for Amumu
When looking at Amumu 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 Amumu. The lane outcome of Amumu consists of three outcomes: Win, Loss or Draw. Let's take a look of Amumu 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 Amumu.
Conclusion
Analyze the available data and learn how to beat Amumu. To master Amumu take also a look on Amumu builds and runes for Amumu. Good luck, Summoner!