Senna has the main objective of calmly outscaling the enemy via her passive, but she doesn't get any AD from leveling up, so she struggles more with being shut down than other champions. That being said, she has trouble dealing with hard engage and all-ins, like pantheon/leona (braum is good too but her attacks bypass his shield, keep this in mind), and this should be used to shut her down early/midgame. Don't let her poke before you gain some kill pressure, then bait something and engage, she'll basically be deleted if you focus her, and if she wasted her heal you can delete the ADC instead.

When i'm playing sup against her I pick pant, wait for the first lethality ingredient (while poking), recall early and then start to engage with killing intent. Pant can outroam her, chip away 70% of her health, tank, and be a better bruiser, I feel like he's a solid pick in this scenario.