I havent played it much but try Fleet Footwork. The reason as to why you'd take it over Grasp or Glacial in theory is because Grasp is generally a bad idea on marksmans (if you're taking any kind of damage you're probably gonna die anyway and you're meant to build high damage and poke from afar as Senna) as for Glacial you really only use it for the AA slows so you can trade easier and kite a bit later on. That isnt really much use imo. Taking Fleet allows you to proc it and then chase forward with your MS and get the soul, it also technically allows you to kite and is actually better in a situation where you're just trying to run away. Plus it deals damage and is easy to proc later.

As for runes I'd suggest perhaps trying the Presence of Mind + Muramana combo. If you havent heard about it then basically you get a lot of AD from it and the mana + reduced mana costs are good on Senna.

After that, Bloodline since the other ones are just less valuable. (you only get 20% of any attack speed you build and tenacity is bad because the second you're CC'd you die anyways basically.) And then Cut Down to help deal with tanks.

As for the other two I've been thinking about either Manaflow Band or Celerity and Gathering Storm.

Shards should be CDR, then AD, then armor since there very likely gonna be an ADC in the lane.