Hello guys! Its misterfirstblood from EUW with another ADC guide for you!
Today i wanna give you a quick overview and explain all the basic Information you need to know about Senna.
It highly depends on the current patch if shes really viable as an AD Carry or better played as a Support, but currently shes not in a good spot right now.
Nontherless, the provided information might help and if she gets some Balance changes it might be a solid pick again!

Because Senna really benefits from initiating short Trades (most likely just using either W, Q and and auto attack or just Q + auto attack) it doesnt really matter how much health you lose while trading because you can heal up within seconds using your Q Ability and Fleet Footwork.
You could for sure argue that Press the Attack or Lethal Tempo would make her better in longer fights but thats really not what senna is meant for so i highly recommend you to go for fleet footwork. It also makes it easier to escape after using your "Burst rotation" since you gain movement speed from your fully charged shot.
Presence of Mind helps you with your mana Problems while laning while Legend: Alacrity and Cut Down optimize your damage output.

As Secondary Tree i highly recommend Inspiration because Biscuit Delivery further increases your sustain and mana and Approach Velocity makes it even easier to get closer to your CC'ed enemies to Q-Auto attack them.

While you can go for a lethality build including Eclipse Muramana and Umbral Glaive i strongly recommend you to focus more on a crit build. In my Opinion it scales better because you will most likely get tons of crit rate for free anyway by abusing your passive.
So Kraken Slayer Guinsoo's Rageblade and Rapid Firecannon build your ultimate 3 item powerspike to burst down opponents aswell as dealing with tanks more efficient than every lethality build could do.

Lord Dominik's Regards The Collector and Serylda's Grudge are generally good items on her in any situation while Guardian Angel Wit's End Mercurial Scimitar and Maw of Malmortius might be very defensive and situational items you shouldnt always buy.

- Try to trade as much as possible but avoid all ins. You really need those stacks to scale well in to late game
- While walking to a lane always check killed jungle monsters for leftover souls to collect with your passive, especially drakes, barons and herals always leave multiple souls behind
- Try to position yourself behind your Supporter/ADC while laning so you can not only poke your opponents or lasthit minions, but also apply the heal to your laning partner/teammates
- By using your E Ability and giving camouflage to your allies you can protect them from on click abilities like Veigar's Ulimate if they lose vision on them
- Always pay attention to the whole map and your teammates icon's to be ready if a global Ult is needed to protect them or finish an opponent.

+ High Burst
+ Very much CC
+ Tons of Sustain
+ Great scaling into lategame if laning goes well
+ Great supporting abilities

- Not much DPS compared to High attack speed hypercarries
- No Dash / Jump makes her really vulnerable if an assasin gets closer to you
- Not a lot of damage against tanks
- Needs a good laning phase to get lots of stacks in early to be super effective in late

P: Absolution When units die near Senna, their souls are periodically trapped by the Black Mist. Senna can attack these souls to free them, absorbing the Mist that held them in death. Mist fuels her Relic Cannon's power with increased Attack Damage, Attack Range, and Critical Strike Chance. Attacks from Senna's Relic Cannon take longer to fire, deal bonus damage, and briefly grant her a portion of her target's Move Speed.

Q: Piercing Darkness
From the twin barrels of her Relic Cannon, Senna fires a unified beam of light and shadow through a target, healing allies and damaging enemies.

W: Last Embrace
Senna sends forth a wave of Black Mist. If it hits an enemy it latches onto them hungrily, rooting them and everything nearby after a brief delay.

E: Curse of the Black Mist
Senna draws the Mist she has stored in her weapon into a storm around her, embracing darkness and becoming a wraith within. Allies who enter the area are camouflaged and also appear as wraiths as the Mist shrouds them. Wraiths gain increased Move Speed, are unselectable, and hide their identities.

R: Dawning Shadow
Senna calls upon the relic stones of fallen Sentinels, splitting her relic cannon into a holy array of shadow and light. She then fires a global beam that shields allies from harm, while damaging enemies caught in the center.

Thanks for paying attention to my guide. My English is not the best so if you find mistakes feel free to correct me.
Greetings
misterfirstblood