Soraka and blitzcrank are both really good counters to Senna. Soraka is a lane bully and Senna is too, but Soraka's bullying is way better since it has more range and she can basically heal any poke you do to her ADC. Blitz lane is super oppressive.

If you dont counter pick her, her weaknesses is typically around lvl 3 onward. She is really strong lvl 1 and 2 due to her strong autos but falls off at lvl 3 when she doesnt get a combat ability, just her E. As opposed to mages or tank supports who need their 3 abilities. Her ult isn't also that stong in 2v2s so 6 isn't that big of a power spike in a 2v2.

She also has a very long auto windup, the best time to use any CC abilities is to hold them until she stands to auto. Some Senna's may even not auto until you use your CC since Senna is so vulnerable to CC due to her low HP and long wind up in order to do damage.