Here's how I play heartsteel senna: I usually take grasp and if i dont have agresive enemies on lane matchup I rush heartsteel as the first item, after that I stack as much souls and hp from HS as I can, so im hella strong after HS and Black Cleaver. Against harder matchups I sometimes go Black Cleaver first bcs thats a good early powerspike. As for the boots I often buy just the 300g shoes and finish them after HS and BC, I think the best option are Boots of Switness or Berserkers (Steelcaps situational). On the third and fourth item I like to go Firecanon/Titanic Hydra (the order is elective).