Senna will try to poke as often as she can with her basic attacks and . If you’re playing as a melee champion, try to minimize your time spent over extended while last hitting or getting warding quest stacks.
Extended trades against Senna will help you in the long run. She doesn’t have the best sustain and her auto attack speed is very low. Capitalise on this by looking for long winded trades when possible.
Senna will run out of mana quickly if she tries to poke often. Use this to your advantage by looking to trade aggressively whenever she is low on mana.
Senna is very immobile and quite squishy. When she walks forward to harass, try to engage on to her as she has little in the way of protecting herself early.
There are only two ways she can land her effectively. You can reduce her chances initially by standing behind minions that are healthy. Do not stand near low health minions as she can still CC you!
Play around her spell. If she misses it, you have a long opportunity to look to trade with her and kill her.
At level 6, Senna gains access to her Ultimate which can be a real game changer in lane as it deals damage and can provide utility. Keep this in mind when fighting post 6 and try to account for the potential shield and damage.
Unlike most Supports, Senna doesn't spike at level 3 as her is purely utility. Use this to your advantage by looking for all in at level 3 if possible.
Senna is quite good in team fights, but she is immobile. Try to kill her first and then look to take objectives like the Baron or Drake.