All right i'm going to get right into this while playing Katarina I advise staying away fromt he fight until you have seen most of the enemy's CC abilities have been used. Then jump in with all you've got starting with a bouncing blade then a shunpo and ending with Sinister Steel and your ultimate. This allows you to do your damage to the enemy team and hopefully score you a few kills so your cool downs are refreshed allowing you to pick up stragglers. This build may not work for everyone but it works for me i prefer my champions to be nice and Healthy by late game. I honestly don't see the point in making a big 5th grade essay out of my builds when most of the things i do with them are pretty self explanatory Runes, Masteries, Summoner spells, etc.
But a lot of times i see people in game and other builds and wonder "why the hell would you buy that?". Well i think most of the items in this build are pretty self explanatory them selves, A lot of the times if you just read what those items do you can most likely see why i'd use them.