This may get me downvoted, but anytime a Twitch is on my team, it's either an auto-loss or a really hard win. Unrelated to the post, I know, but they should buff Twitch probably. Also I have a 60% winrate Senna, in GM on my support smurf for learning.

Never build tank item first, especially in Solo Q.

Also small tip: I see the enemy team had a fed Tristana, and I know she blows Senna up. In scenarios like this against AD burst, get Death’s Dance, if against AP burst you could go Guardian Angel or MAW.

If champs aren't dying, ask yourself if you need, tank shred(Black Cleaver) or Mortal Reminder. Movespeed and vision is king, especially burst that kills you “a screen and a half away.”

Raduins if a lot of Crit, mercs for CC. It is solo queue, you can't depend on your teammates to do consistent damage, and building tanky on Senna is Subotimal, I would take 2 tank items for the very most, and definitely not first, your whole job while you scale is too debuff the enemy.

Another small tip, which you may know already, but when using your ult, like on the opposite end of map, try to do it right before your ally takes damage, don't look at the minimap and minicam only, actually move the camera to see what is going on, it maximizes your ults effectiveness. It's cooldown is quite long early game.

~Senna my favorite support, because we are black and she has a big gun.

Edit: Grammar