Senna will try to poke as often as she can with her basic attacks and Q. If you’re playing as a melee champion, try to minimize your time spent overextended while last hitting or getting warding quest stacks.
Expect Senna to go for an aggressive level 1 play in order to get stacks. Try not to let her poke you down too much otherwise you’ll not be able to trade at level 2 or 3.
Once Senna unlocks her Ultimate R, communicate with your team and tell them that she can impact a fight with her Ultimate R.
Senna has good poke, harass and good disengage with her E. It’s important that you engage as soon as you spot her to make it difficult for her to escape.
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.
As Senna will be going to ward alone in the mid-game, you could use this to your advantage to set up an ambush and take her down. While she is dead, you could try to start a team fight with the numbers advantage or take the Baron/ Dragon.
Senna will look to dominate you in the early game. Expect her to play aggressively at level 1 to gain a health advantage.
Once Senna is level 6, her map pressure and kill pressure increases. Ping once she is level 6 so your team knows that she can influence a fight with her Ultimate R.
The longer the game goes, the stronger Senna’s Passive will be. Try to end it as fast as you can as her late game is quite strong if she has been able to stack effectively.