Hello, I appreciate the feedback, but I usually only focus on updating core parts of the guide that a majority of people will read, specifically the runes section and itemization (in this case, I only updated because of the new build that has become popular). I have not considered updating the abilities section since you can basically find everything about it when you hover the icon (which is located in several parts of the guide), so I do take responsibility for misleading others with "updated to 10.12" (I will be updating this sometime today). Though I'm not too sure what you're referring to by the 'explanation section'...? Senna has not received any significant changes to her kit in the last few patches, so I don't see a huge problem with not putting them in the guide, especially since I don't actually put them in the guide itself or claim to do so.

Glad you found it helpful (:

I'm glad you found this guide useful! All in all I appreciate the input, and I will definitely be trying out Runaan's Hurricane's interactions with different keystones, items, Senna's abilities etc. (a build that has been surfacing in some of my games lately has been Black Cleaver + Runaan's Hurricance vs a heavy AD comp. To me it's strange, but I think I'd have to experiment more to comment.) I do think I've underestimated the potential it can have with Senna, but I've completely disregarded its existence until today.

I don't think I've been using the Domination Tree much, either, since I prefer Sorcery to the other trees most of the time. I could see Ghost Poro working very well, but I followed the Cheap Shot (true damage from CC, I don't think it's a good deal since the damage is relatively small and you don't really need it) or Taste of Blood trend (for sustain? But Senna's sustain is strong with her Q spamming for the most part, so it wouldn't be that great either).

I'll try to get back to you and possibly update the guide according to what I see fit, thank you again for your feedback!

Thank you for the feedback! This is mostly situational/personal preference. I’ve found success with Approach Velocity in certain games, however Cosmic Insight works almost just as good if not better. As a support, you’re looking for chances to spam your abilities, so the cooldown reduction really helps a lot. I had no intention of confusing the reader, but I didn’t feel the need to change it since I explained part of this in the “runes” section. I’ve only recently converted to using Approach Velocity, so I figured I’d only change the first part when you click the guide.

I find denying vision very important, so Umbral Glaive is pretty much my go-to, whether I’m behind or ahead. Additionally, it’s 500 gold cheaper than Youmuu’s Ghostblade. If you’re picking up a lot of kills early, you could buy Youmuu’s Ghostblade. It’s not a bad item, but vision control in my games really helps so I tend to prioritize that. I think Athene’s is really good early on, but I prefer to take it second after getting a lethality item since having a bit of damage helps significantly with her kit (AP scaling is pretty low on Senna so having some damage is a must). Since I get boots from takedowns from Magical Footwear, boots become relatively cheap. I go for Ionian Boots of Lucidity to finish the 40% cooldown reduction cap as early as possible or Boots of Mobility if I'm extremely ahead and plan to roam (sell them later for Ionian Boots of Lucidity once mid-late game hits).

There really isn’t much to the hybrid build honestly, because everything is situational as well. My typical hybrid builds include Glaive into Athene’s, then another lethality item (Edge of Night, Youmuu’s, Duskblade) and finishing off with Ardent Censer if my team can use the attack speed/Redemption/Mikael’s Crucible if the enemy team has a lot of CC/Mortal Reminder if the enemy team has a lot of healing.

As a support main, I think that they don't mean that health is BAD to buy on Senna, but more that there are more valuable stats to get on your items. For an example to a drastic extreme, think about buying Warmog's Armor instead of Ardent Censor. Warmog's Armor grants more gold per stat, and possibly a better ability to boot, most supports would rather build the Ardent Censor, because the Warmog's doesn't advance their game plan. (keeping the ADC alive and fed)
So when they say that "Senna doesn't have a need for the health gained," it's not that health is bad, it's just that other stats such as Attack Damage or Cooldown Reduction serves Senna better.