Eclipse is great with Senna, it's passive works perfectly with her Q + AA combo. Since Senna got buffed to have stronger Lethality scaling's recently it's made going lethality items ideal for her, building 1-3 seems ideal to me, I've found that Serpents Fang and Umbral Glaive are the strong due to their low cost and general synergy with the support role.

Eclipse with Glacial Augment allows Senna to chase enemies basically as much as she'd like to. The extra movement speed after procing Eclipse Passive and Senna's soul gathering passive can keep her around 600 movement speed for basically the entire time she's fighting enemies. Couple this with her range and almost no one will ever be able to escape her.

The shield from Eclipse is nice, but it's not the most important, however it does couple well with shield bash if you're willing to give it a shot.

With Senna support your gold income is usually going to be pretty low, so always try to avoid building expensive items until last/second to last item.

The point of supports is to either Buff allies, or cover their weaknesses. Generally with Senna I try to cover allies weaknesses by building items like Serpents Fang or Mortal Reminder so Allies don't have to. If you can let your ADC ignore anti healing build another more useful item, you're playing Senna properly.

If you're ahead, don't waste your time by going extra lethality items, just go things like Collector (yes I know, it HAS lethality), Infinity Edge, or Navori Quickblade.

Senna is all about getting stacks and items that let her do that faster should be first priority, making items like Rapid Firecannon, Navori Quickblade, and Collector ideal 3/4th items.

Never. Forget. The. Heal.

The biggest mistake I see Senna players make is ignoring their heal and going for flat damage. When you play Lethality Senna the heal is going to be MASSIVE, try your best to use it to hit 2-4 allies when possible, especially if you're going to build an enchanter item.

With Senna you don't really want to be focusing people too much, especially if you have Glacial augment. Generally I find that the best attack rotation is hitting enemies twice to get a soul, and then moving onto the next enemy. This lets you slow multiple people at a time while also getting full use of your passive. Of course this should be ignored when priority enemies are within range, or threats are near you.

Against enchanters, always focus whoever they're going to shield. Serpents fang will reduce all of their shields by 35% which is MASSIVE. If you can properly deny shields you will also deny the enemy enchanter.

Don't just use your E for movement, you can also use it as bait. Enemies don't know how many people are in the cloud so try to always have an ally with you, often times enemies will jump into the cloud unaware that you're not alone.

When Ulting consider the travel time, will players still be in the same place in 3 seconds?

Senna is not a replacement for your ADC (most of the time). Don't take their kills unless you must.

When and Ally and an Enemy are fighting at close range, ALWAYS target the enemy with your Q. Your Q has a larger heal range that extends from either side, so targeting the enemy will hopefully also heal the ally while damaging them.

If enemies are close to a minion at low health you can use your W to kill the minion to instantly proc the root. If you are fighting enemies and someone's about to die, try your best to finish them off with your W so you can snowball kills.

Your auto attacks lower Q's cooldown. If you can, auto attack anything - Jungle monsters, Honey fruit, Towers, Wards, ETC. This can possibly save your allies if they are dying from lingering damage and your Q is only a second or two away from being off CD.

You can target wards to give your Q better placement. Your Q has a huge hitbox that goes out past the target, so using wards you can snipe enemies or allies from farther away.

Krugs and Birds are your best friends. They often drop 3-7 souls. Stay with your allies while they clear them.

It isn't ideal but you can use your Ultimate for vision on objectives if you need to.