Senna scales naturally; build amplifies range and poke.
Spectral Sickle
Évolution Sickle, wards + gold for Senna support
Bonus range on top of Senna's already long passive range
Sheen proc on Senna's slow but powerful AAs
Boots of Dynamism
Quicksilver
Mobility to reposition constantly and collect the souls passive.
Extra shield magic bursts during while she harvest his souls at range.
Reduces incoming damage during his sorties in lane to collect the souls.
Accumulated HP to survive as a pseudo-carry scaling via his souls.
the W Curse of the Black Mist gives him a shield, amplified by Shield Bash to trades.