Hello, I am an ametuer League player and I like to call myself a Senna main (I have mastery 6 on her and have been playing her for a long time). I have experimented with a lot of builds with Senna which all worked effectively but in my opinion Press the Attack was the most fun and powerful because of your Auto, Q Piercing Darkness, Auto combo which will annihilate your enemy in seconds. I like to try lots of builds people wouldn't think of, like for example, I've created a Grasp of the Undying build for Support. Enjoy and have fun in game with my guide!

Pros

She has alot of Sustain in lane
She does a lot of damage late game
She heals allies a chunky amount of health late game
Her Ult is very powerful if used properly

Cons

Doesn't have much mobility in terms of running from fights
Not very good early game
Quite Squishy late game (unless you use grasp)
Very weak against Hard cc

This rune is very good against less squishy enemies like Blitzcrank, Nautilus or Swain (depending on how many Soul Fragment with Ravenous Flock he has). How To use Press the Attack effectively is if you Auto, Piercing Darkness,Auto. Because the Piercing Darkness DOES proc the Press the Attack AFTER you hit the first Auto.

Dark Harvest makes Senna a beast in the late game, becasue her Absolution will also scale and give you more Attack Damage. This will casue your Dawning Shadow to potentially deal 1000+ Damage to Champions. Not to mention that Dark Harvest Deals more damage to champions under 50% Health.

Glacial Augment is good becasue of Senna's low mobility and useful to help your ADC catch up to your Target.

Grasp of the Undying is a strange and experimental pick for Senna, but, it works because of her Life Steal Late Game and because it enhances her Auto attack to deal more damage. And of Course it increases your Maximum Health, so building Tank DOES work.

Now I'm going to talk about the other parts of the Runes:

Presence of Mind Because she'll gain more mana regen Late Game.
Legend:Alacrity Because her Attack Speed isn't the best because of her gun wind-up.
Coup de Grace Because of Press the Attack meaning it will make killing your target a whole lot easier.
Cheap Shot, making it so when you root your opponent it will deal bonus True Damage.
Ravenous Hunter will increase the amount of Life Steal you get from your Piercing Darkness.
Magical Footwear makes it so you recieve your Boots at 12 minutes (unless you get multiple takedowns, which decreases the time at which you recieve them)
Approach Velocity allows you to move faster toward rooted opponents.
Eyeball Collection will increase your Adaptive Force, resulting in your final damage being very good.
Biscuit Delivery: because you won't build Tear of the Goddess as a support, Biscuit Delivery is crucial Early Game.
Font of Life: I Chose this because if/when you root the enemy with Last Embrace then your ADC will heal from attacking the target.
Bone Plating: This is a good choice because if you're backing away from a fight - but you still want to fight, bone plating will activate and decrease incoming damage.
Revitalize is useful because you can heal yourself a little more and same with your ADC.

I don't think I need to tell you why Flash is important, but I guess I will anyway. It's useful to get out of a sticky situation or to engage and enemy who is, for example, in Dragon Pit, you can easily Flash through the wall and swiftly execute your opponent. Heal is always helpful for an ADC because they can't heal themselves. But, because Senna can heal herself with her Piercing Darkness, I use it more to as an emergency burst of health or to heal my low health Support in a time of desperate need when Senna's Piercing Darkness isn't quite enough. Exhaust is the standard spell when going Senna Support because of her low mobility. Ignite can also be used, but if you want ot finish off a target on low health, you can just use Piercing Darkness through a minion or Stealth Ward.

Senna's Items change COMPLETELY depending on the runes. Here is how:

Your First main item for this build should Manamune, which will quickly build into Muramana which will increase you mana by a worthy amount. Second item Should be Boots (the recommend boots are listed above). Third Items should be Divine Sunderer for it's passive which increases the damage of you Autos after using and ability. Your Forth and final Core item should be Essence Reaver for the same reason as Divine Sunderer. Situational Items include: Navori Quickblades for shorter cooldown on all abilities, but especially your Piercing Darkness which, everytime you use and Auto Attack, it decreases the cooldown of it by 1 second. Infinity Edge for more Crit Strike Damage and Crit Strike Chance (which you will have 100% mid-game). Rapid Firecannon for dealing with more ranged opponents like most mid-laners or Tristana. The Collector is useful if you haven't got enough gold because of low CS or lots of deaths and it executes at 5% of the enemies Max Health. Mortal Reminder to fight against someone with high Life Steal.

The items for Dark Harvest. The First item is Manamune because of the same reasons listed earlier in the "Items" section. Then get some Boots of your choice (listed Above). Your Third Item should be Duskblade of Draktharr because of it's passive gives you more damage to deal with squishies VERY fast and go invisible after takedowns. For situational items, then it's the The Collector which will give you some killing power because of the 5% max health execute. The next Item will be Lord Dominik's Regards to enhance your damage on tanks and bruisers (if any). Umbral Glaive if you are struggling with enemy wards and want to get rid of them (even though you could probably get a Oracle Lens instead, up to you). Youmuu's Ghostblade because Senna doesn't have that much mobility. Edge of Night in case CC is a big problem - which it usually is as Senna. Finally, Serylda's Grudge to slow your enemy so you can catch up to them easily and kill them.

First you should build Divine Sunderer to increase your Auto attack damage after you Heal an ally or hit an enemy. Then get Boots (listed Above). For your Third Items, I'd recommend you build an Umbral Glaive to destroy as many wards as you can with Oracle Lens. Then get Stormrazor to slow enemies and catch up to them (along with your Glacial Augment). Finally get Navori Quickblades to decrease the cooldown of your Piercing Darkness to heal more allies at once. For the Moonstone Renewer build.To Start, you will begin getting Moonstone Renewer to dish out heals in combat, then get Ionian Boots of Lucidity for more ability haste. After that get Navori Quickblades so in endgame: 2-3 autos will get your Piercing Darkness back to heal or to damage or both! Runaan's Hurricane is a bit of an unconvential items for Senna but it's good for late game to hit multiple people with your autos. Muramana will give you more On-Hit damage and give you more mana. finally get Black Cleaver for Armour Shred.

To Start, you will begin getting Iceborn Gauntlet to slow enemies and for more Max health and defences. Black Cleaver is for Armour Shred and to give you more max health and Attack Damage. Sterak's Gage to give you more sustain in a fight and for more Health and Attack Damage. For situational items, you could take: Chempunk Chainsword in case you go against a champion with Life-Steal like Gwen or Yone (they usually build Immortal Shieldbow). Titanic Hydra for more damage and Max Health. And finally, Guardian Angel because you will have lots of health to restore once you die.

Her Passive Absolution is one of the best things in her kit, if you see a Mist Echo then pick it up no matter what. They are crucial for your scaling. They grant Attack Damage, Gun Wind-Up speed, Crti-Strike Chance and Life Steal if you have over 100% Crit Strike. A less known fact about Senna's Absolution is that it will steal 10%/15%/20% of enemies move speed.

Her Q Piercing Darkness is the second most vital ability in her kit, it fires a beam of darkness that must travel through a target first to hit enemies. It also heals herself and allies targeted. Every time you Auto Attack, its reduces the cooldown by 1 second. It also scales 180% with Lethality

Her W Last Embrace is important because it's her only form of CC, It's a root skillshot that travels very slowly. Once it hits an enemy it will take 1 second for the root to activate. It does a surprising amount of damage so don't underestimate it.

Her E Curse of the Black Mist is, in my personal opinion, not very good unless you're at the end of Laning phase and roaming around the map with your team, to chase enemies or to run awat from combat. It is a very useful ability and can get you and your team out of some sticky situations!

Her R Dawning Shadow is my favourite thing in the game, I fires a global beam that damages enemies and shields allies in the radius. The shield scales off the amount of mist echoes you have. It damages enemies in the inner beam and shields allies in the outer beam.

ADC Press the Attack
First-Things-First, you will buy Tear of the Goddess and 2 Health Potions. On your first back, you'll buy Boots.
ADC Dark Harvest
First-Things-First, you will buy Tear of the Goddess and 2 Health Potions. On your first back, you'll buy Boots.
If you are leashing the junglers red brambleback or blue sentinel a Mist Echo from you Absolution will spawn. try to ingore it and DON'T pick it up because otherwise you will miss some CS. While in Lane, you want to get as many Tear of the Goddess procs as possible as long as you stay ABOVE 100 mana. You shouldn't get too aggresive because then you will just die and miss out on CS. poke the enemy with your Piercing Darkness as much as possible as well to keep them at bay. I they aggress on you then you should root them, get back a bit, and shoot from as far as you can to win the trade. Paying attention to the map is only necessary if you reach Level 6 and get Dawning Shadow to finish off low health enemies cowering behing tower or running from battle.
Support Glacial Augment and Grasp of the Undying
First-Things-First, you will buy Spectral Sickle and a Control Ward or 2 Health Potions.
You will Walk to Lane with your ADC (unless you're leashing). keep poking and using your biscuits from Biscuit Delivery. Root when you can and push in when your ADC does. Keep getting Mist Echoes to increase your healing and damage. Use you Dawning Shadow to protect allies from all-the-way across the map to get the edge in a 1v1.

ADC Press the Attack and Dark Harvest
You should begin leaving lane when you have your Second or Third Item to roam and help out your team in other lanes. For example: Your Mid Laner is pushed up against turret and it in need of assistance. Go in from behing your target, Root them with your Last Embrace then demolish then in a matter of seconds. About when you get your Third or Fourth Item Teamfights should begin. When you do, land a root on an enemy, then stay in the Backline and shoot your targets from afar. Then Rinse-and-Reapeat. Make sure to target the enemy you rooted, because it's likely that they are a Frontliner, and in doing so, you will reveal the Backline for you and your Allies. Try get Dragons whenever you can (especially Fire Drake, because they'll increase you Attack damage). Make sure the enemy aren't Back-door-ing your Tower or they'll very quickly get to your Inhibitor.

Support Glacial Augment and Grasp of the Undying
Stick with your ADC for Early Grouping, if you don't the enemy will see you as a major threat and go for you first. When entering a lane, make sure to use your Curse of the Black Mist because, with allies or not, it'll make your opponent think that you'll be hiding teammates in your Mist. make sure to top up your allies Health when you can. If your running Grasp of the Undying then you want to Proc the effect as much as possible still.

ADC Press the Attack and Dark Harvest
The Late Game isn't much different from the Mid Game... Because there are lots of similarities between the two: For Example, If you run by your self without your teammates then you'll probably die a slow, lonely, painful death. The base message for this is stick with your teammates use your Curse of the Black Mist whenever you can to hide your real number of teammates in the Mist. When in a Team Fight: first try and exploit the range of your Last Embrace to root your target from the furthest distance so you and your allies can rush in and, hopefully, destroy the frontline in order to get access to the squishy back line. No matter how fed you are, remember you pretty much a glass cannon to don't roam alone. Once you have full build, make sure to grab an Elixir of Wrath to boost your Life Steal and Attack Damage.

Support Glacial Augment and Grasp of the Undying
As the Support, obviously, you should heal your allies health to full when possible. But to ensure that you use your Piercing Darkness to it's fullest, if your with friends, ask them politely to line up for you can heal all of them in the line. Like I mentioned before in the previous paragraph, remember to use your Curse of the Black Mist to hide your teammates when entering a lane or where there are lots of enemies. At this point your Piercing Darkness heals about 250-ish, with that in mind, keep allies topped up on health.

General Notes for Both Builds:
Get dragons when you can with teammates (or without, you can probably solo it at this stage in the game). Get Baron Nashor when you Ace or are in a position of power where the enemy team can't interfere. As the for the elder dragon get is as soon as it spawns, Same as when getting baron nashor .

Thank you for taking your time to read/see this guide, this is my first guide so forgive me if it's not very good. if there's any counters to Senna you can think of, leave a comment and i'll add it to the Threats section. and likewise for the synergies. I would like to thank my regualr friends and teammates for helping me when I test these builds.

when this guide go thirty-five THOUSAND views I was so pround of myself, I want to
thank you SO much for seeing my puny guide compared to all the others on this site.