The current meta is to build Senna just like other ADC's and stack attack speed. This doesn't play to Senna strengths! With this build you will be a slow shooting cannon but your AA-Q-AA will halve enemies health!

We start with long sword, so we have a disadvantage at the duels. Thats ok, since our counter attack from tower is really strong. If enemies get cocky and attack the tower W the minion and root them instantly. If you W them directly and they run away it will still be enough for a good trade.

Once you get sheen, try to get an AA after Q. W-AA-Q-AA will be strong enough to kill the ADC, as long as you are healthy enough at the start and get some help from support. If they have a high damage support its ok to give up on the second auto, so the trade balance doesnt go wrong for your side in the end. If you clip the auto to attack the support, it usually won't end well.

Key is to stay at 1 death max in this stage, past 20 minutes you should be more valuable to your team than the enemy ADC even if you are at 1 kill disadvantage. We play for the long game!

As usual, fight for objectives, if its low elo a lot of the game will be in mid where you can do solid poke from the back line, stack souls, and hopefully catch some people under tower with W. Try to help with the dragons cause they give you 2 souls. It's usually ok to take some jg or even follow the jg around cause monsters drop more souls than minions.
Use teleport to get an item advantage prior to big fights. Don't over step with destination cause if you get CC'd you will die. Help your melees catch up to the enemies with W/glacial. With your Q they can tower dive more safely.
Keep an eye out for close duels and decide them with R.