I've been playing Senna for a while now, and throughout my games I've found her lacking in certain aspects. In most fights she's very squishy and barely gets to use the items I get in a standard Lethality or Support build.

To compensate for this squishiness I've wanted to build her more HP oriented, but since I didn't want to lose out on my damage potential I had to find items that synergize well in that aspect. Thus, this build was born and so far it has close to 100% win rate.

The advantage of an off-meta build is people won't know how to counter it. With a tanky, damage-dealing support you usually pack enough of a punch to ensure a victory. You are too dangerous to be left alive or you'll simply blow the enemy away, but with tons of HP and survivability from runes and synergies you don't just die. This gives your team enough time to kill the enemy should you still find yourself in a sticky situation.

Runes:

I chose Resolve since we are an HP focused build. Grasp of the Undying and Overgrowth are both there to gradually increase your HP pool throughout the game. Font of Life brings the support aspect to this since your entire team will benefit from your HP stacking. Second Wind is to provide some additional sustain.

In secondary I pick Magical Footwear because Senna can really use that speed buff, and it saves you 300 gold. Biscuit Delivery is for early game lane sustain so you can stay there until you have the 1100 gold to get your first key items.

Then simply 2x Adaptive for that little extra AD early game, and of course Health in a HP build.

Items:

This entire build is centered around three items. Divine sunderer is there to provide your damage and first batch of HP. It will also give you some much needed armor penetration as you build more items. Manamune is for some much needed mana sustain and your first power spike. Titanic Hydra is a logical choice for any HP/Damage build.

From there Sterak's Gage will provide you with the survivability you need against full build enemies, just as Black Cleaver allows you to shred the armor of their tanks. Providing a teamwide damage bonus to any enemy that is affected.

Situationals:

Umbral Glaive: Use this when facing a Teemo support cause this baby will save your life! It detects his mushrooms and that will save you a massive headache.

Edge of Night: Consider this item when you find yourself struggling against the "hooker" supports. If the enemy has a fed burst champion like Katarina then getting grabbed by these is a surefire way to die. In this case, a spellshield might be your only hope.

Chempunk Chainsword: This is only useful against strong healers like Dr. Mundo or vamp stackers. The only way to shut those annoying enemies down is to cripple their healing and this item does the trick. If you find that your teammates aren't building any grievous items then you should grab this, otherwise stick to the original build.

This is where Senna is the most vulnerable so thread carefully. The most important thing is to not die, once you fall behind it's very difficult to regain that momentum. That being said however, Senna does has the potential to be a very aggressive support. She can poke with her Q and those roots will keep enemies at bay or help secure a kill.

Start off with a Black Mist Scythe as your standard support item and just bash away at the enemy to collect that gold.

First back should be at 1100 gold to buy a Sheen and a Tear of the Goddess. This will up your damage quite a bit and help with the mana issues you're bound to experience. From there build your Divine Sunderer and pick up your boots as soon as they're available from your runes.

For your first damage spike I recommend going for Manamune, since you'll have been stacking your tear. You're still a bit too low on max HP for Titanic Hydra to provide you with a decent boost. With your Sunderer and Manamune you should have enough firepower to dominate the enemy squishies.

At this point the game should have progressed into its midgame phase, and you'll be focusing on skirmishes and objectives. Make sure to keep dragon warded since this is the most important kill in this stage of the game. In fights you should focus the squishies, their assassins and ADCs, if possible.

Once you get your Titanic Hydra it's showtime. You should have plenty of HP stacked from your runes to get a significant damage boost. From now on it's easy sailing and you can start taking on their tanks as well. Make sure to keep using your abilities to support your team and don't try to take on the enemy 1v1, you are not a carry after all.

Should you find yourself caught by their assassin, sure give it a go. They won't be able to burst you down as easily since their lethality builds lose their usefulness against a huge stack of HP without any armor. And once you get Sterak's Gage they won't be able to hurt you anymore anyway.

By now your build should be almost complete and you'll have a fancy score to show off as well. This is the most crucial phase of the game because you are now susceptible to get tilted. Your giant pool of health and your impressive score can make you cocky enough to lose the game.

Keep paying attention to your positioning, don't engage 1v1 fights unless they are forced on you. Keep supporting your team and work those objectives. Since you will still have your upgraded Scythe, make sure to put a ward on Dragon and Baron as often as you can.

You will find that your enemies will be quite scared of you. The only real danger to you right now is their carry and their tank. Since the carry can quite easily rip you apart, and the tanks can, well, out-tank you because they actually build resistances and you didn't.

Nevertheless, your Sunderer should provide you with enough armor penetration so you can still do some decent damage to them in teamfights. As usual you should try to focus their squishies since that is where you shine, take away any damage advantage their team might have. But do NOT sacrifice your positioning to do so. Remember your Q penetrates and if you use it on their tank it will likely hit their carry as well.

Keep rooting, keep healing, keep dealing that damage and you will find this is a surefire way to victory. Once you have enough gold to pick up your last item, consider selling your Scythe if visibility isn't going to be a real issue, your two free wards are going to be enough to ward either Baron or Dragon.

To finish this off I'd like to leave you with a few tips & tricks to improve your average gameplay.

Senna's ultimate:
This is one of the few ultimates that still has a global range. It doesn't deal that much damage but it can be enough to snipe a low health enemy that just escaped. And you can do this from anywhere on the map. Not only that, it also heals your allies so even if you don't hit the enemy, that little heal might be enough to tip the balance and get you an assist for your effort.

Senna's Q:
This little beauty is your bread and butter. Since it penetrates enemies you can use this on enemies or allies and hit the one behind them. Use this to your advantage early game to rack up that gold from your Scythe.

Senna's E:
Don't forget your little mobility buff also grants movement to allies. This little piggy is enough to get you an assist if your ally manages to get a kill after his movement has been boosted by you. And since it counts as a spell it will also trigger your Sunderer's spellblade. Combine this with your W to kite enemies chasing you for a kill, they'll be surprised to see that damage from someone right outside of their reach.

I hope you all try out this build if you are a Senna main, or even if you aren't and get autofilled into support. Let me know what you think of it when you do and leave an upvote if it worked for you!

I'd like to also mention that I'm a relatively low ELO player, so this build might not be able to perform as good in the higher ranks. But considering the majority of players reside in silver, this still holds up for most people to at least not throw those games where you get autofilled as a support and don't know how to properly play one.