Senna is going to be released on 10th November and in this guide I will show my ideas on how to build, play and win with her. Since there isn't too much info about the her yet, I will update this guide based on my game experience on the live servers.

SUMMONER SPELLS - FLASH: You can use Last Embrace and then Flash to cats it from your new position, taking reaction time away from your enemy. But since Senna is a very stationary champion, you should only use it if you get somethign from the kill. (Item, Objective etc.) Heal: If you are playing on support and your Adc is playing Teleport, then Heal is probably a better option than Ignite. If you are playing Adc yourself, I recommend taking Teleport.

In lane you want to constantly auto attack the enemy whenever you can to get your passive Absolution and Spellthief's Edge stacked up. After attacking an enemy champion Absolution will apply Mist on them for 4 seconds. If you then auto attack them again Senna will collect the Mist gaining 1 Attack damage per Mist and after every 20 stacks she also gets 25 range and 15% crit chance (35% of crit chance above 100% is converted to lifesteal).
Mist Wraiths may spawn from dead enemies, and Senna can kill them for 3 gold and a Mist stack. This is scaling infinitely, making her a strong late game champion.

Senna's abilities have high cooldowns so you want to use them when you really get value from them. Piercing Darkness heals all allies and damages all enemies hit by the ability. It can be cast on Champions, Enemies, Plants, Wards and Turrets. Creating huge playmaking potential for big aoe heals/damages. Especially when gaining access to Wards from Eye of Frost to cast Piercing Darkness on a higher range since the cast range is lower than the range of the ray.

With every auto attack the cooldown of Piercing Darkness will be reduced by 1 second, so cooldownreduction and attack speed are valuable stats. The lower the cooldown of Piercing Darkness is the more impactful will 1 second reduction per auto attack be.

However it's manacost is very high with 110 at rank 5 which you would get on level 9 where Senna has 710 Mana (960 with Manaflowband). Senna has mana problems so The Blue Senna Build is a good way to go. (Although it also gets a little red later on)

After completing Frostfang you will buy and either Mobility Boots for roaming or Berserker's Greaves for skirmishes and Tear of the Goddess to start stacking it up. Due to Sennas high cooldown's, it's going to take some time, however that is why you will rush Manamune as next item. It can be stacked by also auto attacking, which means you are not only relying on your spells.

The next item then is Essence Reaver, it's giving you cooldownreduction and also mana for critical strikes, by the time you complete this item you'll probably have atleast 40 Mist, making a total of 55% crit chance with and the power spike from Muramana on top of it. From that point on Senna is really strong and it will only get better. The most important thing will be to get into fights and auto attack from safe distance to constantly reduce Piercing Darkness' cooldown and land multi-heals.

The next items will be:

Rapid Firecannon: Combined with her high range, it's basically free damage which will reduce Piercing Darkness' cooldown.

Runaan's Hurricane: Against melee champions you should buy it before Rapid Firecannon. The thing about this item is, that every bolt from the passive counts as an autoattack, if you are attcking 3 enemies, it will reduce Piercing Darkness' cooldown by 3 seconds per hit.

Remnant of the Watchers for Support: You can't renounce the Wards, they're too important in the game.

Infinity Edge for Adc: You can also buy it one Support after on of the attack speed items, depending on how useful the effects would be. With Absolution you will probably have 100% crit chance really early, making Infinity Edge a good item for Senna.