League of Legends (LoL) Question: How do you play Senna?

I played a lot of Senna upon her release. I think I know a bit about her to help.

In lane:

You are essentially a Caitlyn and Jhin hybrid play-style wise. You want to poke out the enemy as both a marksman and support by farming your Absolution. You can land an auto-attack or any ability, then right-click a champion to farm mist. It's a plus to position Piercing Darkness to both damage and heal your carry. It's still beneficial to heal allies during downtime (ex. unsafe to trade, carry is low). You can also farm mist with your Piercing Darkness.

If you're a marksman, creepscore is pretty difficult. Your mist will despawn quickly and your auto-attacks are horrendously slow. My advice is prioritizing your creep-score over mist since it's more profitable in the long-run. You need to learn your auto-attack wind-up and determine if farming mist will mean missing minions; if it does ignore the mist. If you want to try out Senna for your first time, play her support. She is much easier to execute without managing the minion wave.

Build:

I've found two builds to be successful, while both are unusually different, give her great strength. I personally find her stronger as a support since in the marksman role her damage can be easily countered by tanks and she is vulnerable to diver types that can continuously stick into her Curse of the Black Mist range. However, her support healing will always be useful and welcomed in a multitude of ally and enemy compositions.

In the support role, straight-up traditional support gives you insane healing. You want to upgrade into Frostfang then get items such as Athene's Unholy Grail, Ardent Censer, Redemption, etc. You can add some damage items if you'd like such as Infinity Edge late-game to amplify your crits or Mortal Reminder for grievous wounds. The reason an enchanter path works is her healing/shielding output is ridiculous and mist will give enough damage stats.

In the marksman carry role, Muramana is a surprisingly great first item to preserve mana. I like Duskblade of Draktharr as well since your Curse of the Black Mist triggers duskblade's passive. Afterwards, it's important to know what not to buy. She has no scaling attack damage per level poor attack-speed per level, so attack-speed/sheen items do not give her good stats. The critical strike conversion to life-steal is also bad scaling. I personally like lethality items (such as Youmuu's Ghostblade, Edge of Night) and an Infinity Edge to amplify crit damage.

Senna is great with Boots of Swiftness in all regards. She gets movement speed in her Curse of the Black Mist and between auto-attacking, maintaining these movement speed buffs is crucial to positioning and kiting. It's a similar situation to Jhin who may also build swiftness boots.

Runes:

Senna is good with Summon Aery as support. It plays towards your poke playstyle and gives you access to many useful secondary runes (my favourites are Manaflow Band, Celerity, and Absolute Focus.) It gives you more utility to protect your carries too (this is better than getting Kleptomancy to hit items sooner: see the next paragraph for kleptomancy).

Senna is good with Kleptomancy as a marksman carry, or if you decide to build damage in the support role and want early access to lethality items. It works because your Piercing Darkness applies on-hit effects so Q > AA consumes both charges of kleptomancy in addition to farming mist for your Absolution. It enables you to extend the lane phase a bit better than sorcery.

Overall:

You win by playing to your maximum range. Once you input any auto-attack, Piercing Darkness, or Dawning Shadow, take advantage of your passive movement-speed to reposition. If you can master this technique, you will do great on Senna.

AD is the only way to build her , you can go for an ardent or something but that willnot benefit you as much if your team is not playing well. if you want a pure healer/utility support or supports that synergize great with ap items try Yuumi, Soraka, Janna or Lulu.

and about poke, if they dont have a lock CC combo you want to poke them as much as possible, but if they have something lik thresh, or anything that can easily catch you and keep stunned for a good amount of time you want to poke just with your Q and focus more on peeling the adc, most of times a stun on you early game is equal with death or flash, especially if your adc is not really aware what is happening right in front of him.

Since you can actively play her both as an Adc and a Support. Your playstyle differs depending on that style. If it is as an adc, then focus on csing properly, using Q for poke & collecting mists. If it's for support, then actively try to poke with Q, Stun with W if the chance is there & always save your E when the enemy jungler is trying to gank, as it's a great tool to get out of most situations. Senna doesn't have any ways of healing their laner, the only ability that actually "heals" them is her ultimate, which is just a shield, you won't be using that all of the time for your allies.

Currently, I've seen most people building her with Spellthief's Edge > Remnant of the Watchers > Duskblade of Draktharr > Boots of Swiftness > Youmuu's Ghostblade > Ardent Censer into an Infinity Edge / Redemption or Guardian Angel for the remaining item slot depending on the situation of the game.

I believe Arcane Comet is the most used one for her, with Manaflow Band, Transcendence & Scorch. Secondaries are Resolve with Revitalize + Bone Plating.