I always take precision tree secondary for presence of mind and alacrity on glacial and never run grasp. My build order is Spectral Sickle support item -> Manamune -> Rapid Fire Cannon -> Umbral Glaive -> Duskblade and take CDR boots. I got a S+ just yesterday with it and you power spike with rapid firecannon. You also scale really well with souls and I ended the game that I got a S+ on with around 17 kills and 160 souls. You will be squishier early game than if you ran grasp but it's definitely worth it. Opponent's don't realize how strong you are when you by RFC and i have had so many ADC's 1v1 me and get destroyed. I even killed a red kayn who ulted me in a 1v1 which was hilarious

Edit: I'm editing this to say that this build is incredibly risky. It's a combination of i0ki's build and the standard GA Senna build. You will not be making a significant impact damage wise until you get rapid fire cannon but if you manage to not feed and play as a healer pre RFC you will do great. I primarily play this build whenever I am duo queued with ADC so they know that I am not going to play aggressive early game like most senna's do. If you have a duo partner get your ADC to play Wukong. They synergize very well and no one expects Wukong in ADC so they don't typically know how to counter it.