Senna can be played both as supp and ADC and both ways will have a great damage thanks to her passive (her souls) and the build you make with her, so we are going to comment champions to counter Senna.
Champions counter Senna
Tips for facing Senna
Senna is a very weak champion so you can make a great engage with a simple kill so we will try to play aggressive champions (if we know it is a Senna) to make life impossible, let’s go with the champions with which to counter Senna:
A great champion for this is Tristana, because she can jump over Senna with her W and when you apply her E you will probably leave her out of line and if your support has been able to hit her you may have killed her easily, it is unplayable with Senna a line against Tristana.
Thanks to Senna’s lack of mobility, Leona makes her a very strong pick against her, you will be able to do a CC chain without any problem and probably finish her life by yourself without the need of your ADC since Senna’s base life is very low.
The ease you have to hook Senna is incredible, and if she plays behind the minions you can always approach her with your W and hit her with your E and then your Q, her base life is very low and you can easily finish her off.
If you want to play something more passive you can play Nami, since you have CC and heals to be able to kill Senna with a little poke.