Who am i?
I'm a Brazilian Gold IV with 340k mastery point on Senna. I have a safe playstyle, this guide is for you that wants to have a decent amount of damage while still play safe. Sorry for any language mistakes.

Why choose this build?
All core items: Locket, Galeforce, Youmuu's, EON and Glaive interact with Ingenious Hunter. They're cheaper and offer a stronger early game with Dirk or Life + MR + Armor (if you're building locket). Youmuu's 1st item will solve most of your positioning mistakes, helps you to escape from ganks, low elo has a lot of useless fights and the extra move speed will help you to reach them before they're over and escape with it's active effect if your team dies. Add EON and the enemy team will engaje on you, you won't be stuned and you can escape with active, that makes you really safe while still have a decent amount of damage. This build has a great pick off potential with Senna's E + active + auto attack + W.

Where are Kraken Slayer, Divine Sunderer and Guinsoo's?
Although Senna is stronger at late game, early game is the moment you have to create advantage on your enemy (or not let them snowball you) with less interference of top, jungle and mid allies and enemies, stronger early choices will help you with it. Kraken Slayer and Divine Sunderer are the most common items on Senna but both are too expensive, Noonquiver is weaker than Serrated Dirk and neither Phage, Sheen or Kindlegem are good 1st back items on Senna. Although Guinsoo's is a good damage item, it lacks of utility.

Youmuu's Ghostblade Vs Umbral Glaive 1st item?
If you're on a higher elo you may wanna rush to stack Zombie wards earlier. On lower elo your positioning mistakes may be a bigger problem than vision control, in that scenario is a better option. Remember that might be a useless item if you're being stompped on 3 lanes, you won't have the chance to enter in your own jungle to control vision.

Doesn't it lacks of mana without PoM?
Initially I thought that too, i would be lying if i said that i never lacked of mana, but it is really rare. Biscuit delivery fulfills almost all mana needs.

Damage vs Tank Senna?
and are usually enough to survive is most of the situations, Tank Senna is not mandatory as it used to be, but still be a better option if the enemies can kill you in a blink of an eye before you escape with active.

Optional Runes:
Use whenever you want to buy boots early in game to dodge skillshots or have more mobility to escape from hooks.
Another viable option to reduce more itens and summoner spells cooldown.
Use if you're not successfully stacking zombie wards ( look at rune section on your match history after a few games ), you'll have a stronger poke on lane phase.
Another viable option, but all your core itens have cooldown, ingenious hunter is probably a better option.

Take it to have more lane pressure
Regenerates more mana early game than , so you can hit more
add damage to your

Optional Items:
Whenever there are a lot of tanks on enemy team.
Whenever there are a lot of magic damage on enemy team.
Whenever you think you game may be decided on a fight at jungle where vision control is too important.
Whenever you need grievous wound, i suggest you don't buy the full item and sell it later when your team buy their grievous wounds.
Whenever there are a lot of shield on enemy team.
Personally i don't like RFC, but build it as you are used to.