Deal more damage to champion with more maximum health than you.
Offense
+5 AD or 9 AP
Flex
+5 AD or 9 AP
Defense
+6 Armor
WhySummon Aery
Senna ability to deal repeated offensive damage with auto's and defensive heals with Q allow you to make excellent use of Aery
WhyManaflow Band
Manaflow allows you to sustain your mana pool in lane with the guaranteed Q hit.
WhyAbsolute Focus
It's fairly easy to remain high health as Senna thanks to your Q's healing, allowing you to make great offensive use of Absolute Focus, especially in lane.
WhyGathering Storm
Gather Storm is your lane game insurance policy.
WhyLegend: Bloodline
Bloodline allows you to sustain thanks to your high base damage.
WhyCut Down
Senna has very low base HP so you'll be making constant use of Cut Down.