FIRST THINGS FIRST

This is not a good guide for a new Senna player. While this build can be used as Support, This is a guide more suited to Midlane or Botlane Carry. There are a number of decision points where an experianced player will need to make choices based on an advanced understanding of the game.
If you are interested in trying this Top Lane, You will DEFFINATLY want to get Swifties or Zeal early and your results may vary, but please, let us know how you fair.

Lets begin.

THE MISUNDERSTANDING
A critical componant of this build is that there are two(2) significant misunderstandings about Senna. The FIRST is that Attack Speed is "Bad" on her. While I won't say that Attack Speed is GREAT on Senna, Bad is a CLEAR misunderstanding of how Senna's Auto Attack works. Without going into all of the math on how AS Level growth works the short answer is that Senna gains a reduced increase benifit from Attack speed and Auto Attack wind-up. To understand why this isn't as bad as it sounds, it becomes nessissary to point out that there are 3 componants to an auto attack. Windup, Attack Speed and Missle speed/distance traveled. Yes there is also damage, That's also a very complicated, and seperate, conversation. It's this third componant that chages the rules. Senna is the only non-melee champion with a Missle speed of Instant. The games depreciation of AS on her is a direct balancing to this powerful attribute. Because of this, "Out of the Box" Senna is already at a speed advantage over her Marksmen counterparts. It just doesn't feel that way because most players fire and forget while Senna stayes on target. This guide is not here to tell you that you should build AS on Senna, and I won't, but I will tell you stop looking at AS as "Bad" choices. Kraken Slayer is a real option on this champ (though if you're killing tanks, go with Divine Sunderer).
The Second misunderstanding is the belief that Senna is a back line sniper support. Shes great at it but she is more versitile than that. This guide will show you how to be a mid ranged, tank killing, wave clearing machine of death.

THE DIFFERENCE
I'm not going to go into the long math. But lets just say that when you chose item paths for Senna, there is a specific descion point. Which ZEAL Item do I build? As a sniper support you would build Rapid Fire Cannon. And normaly I would never fault this as the [Q] synergy is strong and the extra distance is nice. However, It is NOT the most synergistic of the Zeal items on Senna. Runnan's Hurricane is. By quite a lot actualy. Now, There are a number of champions that have unique internal (besides the normal external ones) synergies with RH. Senna's is very impactful and requires specific considerations. In truth, theres two(2). First is the effect on mist colleciton. WIND'S FURY (RH's abiity) will prime AND collect mist from champions which allows for a potential 3 simultanious mist collections. Also, while RH will never target a ghost on the ground, It will fire during a pick up, making run and guns more effective.
And Second (the biggie) Winds Fury triggers Piercing Darkness' Cool Down Reduction. This means that each of Winds Furys bolts ALSO triggers a 1 second reduciton in Cooldown. This means you will be firing [Q] as much as 3 times as often. To be blunt, the effect this has on Senna's Mana Pool is significant and It is this consideration that effects the entirity of the build.

THE RUNES
With one exception, the rune choices here are fairly non negotiable. Most players will look longingly at Cut Down but "It's a trap". You're only looking at it because you've been conditioned to think that Legend: Alacrity is bad on Senna. Every Rune choice in this build was picked to specificly address a Senna weakness or enhance this specific build. I've tried a significant number of variations so If you go exploring, don't blame the guide. The one real negotiation point is going to be Manaflow Band for Nulifying Orb. The Bands strength is in the early game. By the time you get to team fighting Nulifyin Orb...may...help you with mage assassins. The mana regen is insignificant compaired to Presence and the added mana/AD-boost is minor. But, unless you getting a facefull of Syndra/LeBlanc, the Band is a better choice early game.

SUMMONERS SPELLS
This is a Flash build, like most. But by mid/late game you'll be using it offensivly considerably more than you will defensivly. If your mid lane I recomend Teleport. In fact if you can time your first back at 8-900 gold you will see significant returns early. If your top you have some space issues, and that level 4-5 ghank. I would still use teleport but that will be more match-up specific and this is not a guide for new guys. Let us know whats working.

THE BUILD PATH AND THE STRATAGY
This path is not, nessicarily, liniar, nor is it cut in stone. There are a couple of descision points. This is why I wont just lis teh path out but instead talk about the descision points. Gerneraly, this is the build that will have the most impact. The real power of the plan are the first two items. Manamune and Hurricane (and Swifties). These are non-negotiable in this build. They are WHY you are here.

Before I get into the descision points, lets talk about the basic stratagy and why it is. Once you have Runann's Hurricane you will be mad spamming your [Q] every 3-4 seconds-ish. Piercing Darkness Level 5 has a cost of 110mana. At LEVEL 18 Senna's Mana pool is 1115. Thats 10 shots. With no other factors and using no other abilites, you'll be out of mana in 30 seconds. No ultimate, no escape pool, no snare. Practicaly, With other abilities, your looking at about 15 secs untill your dry...at level 18. In order to manage this, It's nessissary to take action PRIOR to getting hurricane. We've got the runes, however this is one of the two (2) reasons we build Manamune first.

START with TEAR, YELLOW WARD and 2 POTS. You will definatly use the pots.

The opening stratagy is simple. Don't die. Expect an assault at level 2 and 3 and a Ghank. The fact that your playing Senna will embolden most players early on. Activly, there are, again, two (2) things your trying to do. CS effectivly and score stacks for TEAR.


Two(2) points (again with the duality) The first significant point of concern is going to be early wave management. Because Senna's Base AD never changes (50 L1 - 50 L18), last hitting can be a challange. If you get shoved its going to get ugly. The good news is that a sustained neutral wave state is perfect for Senna and if your in a constant state of autoattacking, which you should be, this is maintainable. so thats your first goal.

The second point is trading, and this is where the "majic" happens. The objective of this early game is not to get mist or get kills..as nice as they are...it is to stack Tear. This means max range and constant AA on wave. The trick is the use [Q] on cooldown => AND <= love tap the enemy player as often as posible without letting them counter play. This means using MINONS to pivot your [Q] to extend your range to target. While this means you won't be following up to get mists it does keep you healthy and safe.

First Back
Hopefully this is around +900g and you definatly want to wait until after 800g if possible. This may seem low, and it is, but this helps to manage the AD scaling issue. Buy Boots for the ghank-fest coming and ALWAYS buy SHORT SWORDS until Manamune is complete. If AD upgrades are available they are the FIRST thing to buy untill you have Muramana...every time...you need the AD. The only real exception to this is if your in base during the 1050g window before completeing Muramana, don't be afraid to splurge on BOOTS OF SWIFTNESS. Anything else delays something you MUST have but the boots will make managing the team fighting/ghanks/Lux much easier.

Muramana Back
After 2900g (Swifties are another 900g) you should have Manamune completed and at least one tower should have fallen. Without Rift herald it wont be yours. This is fine, as this is where things get interesting in the mid lane and Its likely that your tower is doing just fine. Your next item path is going to be ZEAL => RUNNAN'S HURRICANE. This Item's movement speed + Swifties + runes is going to make your positioning very strong coming into team fight stage. With Zeal at 1050g and Hurricane 900g after that your first 6300g will bring you to a rather SIGNIFICANT power spike and sometime around the Hurricane purchase Muramana is going to complete. There are champions that typicaly counter Senna (Pyke) that will die from an overabundance of confidence.

First Major Decision Point
This is where you really need to take stock of whats happening in game. The most important question at this point is this...Is there a mid range mage (Sylas, Diana, Ryze, etc.) whos been hard fed on the other team? If the answer is yes, you have a problem and it will eat you. The solution to the problem is MAW OF MALMORTIUS. I've tried all the others, the answer is Maw. Get Maw and know happiness. If they do NOT have a Hyper Fed Midrange Mage,the next item is RAVENOUS HYDRA. You will be in team fights and wave clearing from this point out. This will give you more damage, more healing, AOE damage, More [Q} spam, and a little extra healing that we dont care about. You will be forgivin if you cry HEEAAVYYYYYYY BERRIL! when firing your [R] DAWNING SHADOW. Find someting sturdy (tank Geran's woot) to stand behind and [Q] spam untill the game is over.

Second Major Decision Point (why always two's?)
Compleation of Hydra is USUALLY game over. Sometimes it goes on, so heres what you do next. If they have a lot of TANKS you may want to build DIVINE SUNDER at this point. If not, then the best Mythic is ECLIPSE. By this stage your going to be over 100 Mist which is more than 20% Crit so last item can be IE but if theres any armor building going on, and there probably will be, LORD DOMINIK'S REGARDS is the way. This will only happen if your fed or your team has no idea how to close a game.


Good Luck, Have Fun!