Against senna, The longer the game goes, the stronger Senna will be in team fights. Try to end the game as quickly as possible so she is unable to be really effective in the mid-game.

Shut Senna down as quickly as you can in a team fight. This will reduce her ability to poke you down, deal damage and win the fight. If you can’t shut her down quickly, you might need flank her and attack from the side.

Play around Senna’s Ultimate R. If she uses it, you have a long cooldown to abuse her and her team.  Against senna, The more the game goes, the stronger Senna will be in the team fights. Try to put an end to the game as quickly as possible so that she is unable to be really effective in the middle of the game. Close Senna as quickly as you can in a team fight. This will reduce her ability to bring you down, inflict damage and win the fight. If you can't close it quickly, you might need to flank her and attack on the side. Play around Sennas Ultimate R. If she uses it, you have a long cooldown to abuse her and her team.