Hey, my name is Spinning Axes, and I'm a plat level ADC main. I recently picked up Kalista, to find that I'm a natural at her. I'll go in depth of my build, and why it's the best build for her.

Doran's Blade and 1 Health Pot is the average start on an ADC, no explanation needed here. I rush Infinity Edge in all situations because it's a reliable source of damage at all stages of the game. Once Kalista has a Pickaxe and BF sword, she is able to solo dragon if she has 20 seconds to do it. Statikk Shiv works well with IE because it offers the crit chance to make IE's passive useful. It also allows Kalista to be able to duel and pack a punch in each of her auto attacks that crit.

(Enemies have armor)
Last Whisper is a need. A lot of lower ranks don't think Last Whisper does damage, because 40 AD is really minimal to that whopping 70 AD from Bloodthirster. Well, that 40% Armor Penetration will make you deal MORE damage than the Bloodthirster. There is no reason to buy BT if there is more than 3, or even 2 people building armor on the enemy team.

(Enemies without armor)
Bloodthirster here. Not only will you get that sustain everybody loves, you also get massive AD for taking objectives, dueling, and consistent damage in teamfights. Remember, Last Whisper will outdamage Bloodthirster if the enemies have armor, so pick wisely!

(When do I get boots)
I usually get tier 1 boots while making my IE, and finish them before Statikk Shiv. This allows you to hop around a lot more, and a lot farther.

(Defense items)
-Guardian Angel if you're getting dived.
-Banshee's Veil against the likes of Leblanc or Annie.
-Mercurial Scimitar when against Nasus wither, or something like a Malzahar ult. You'll want to break free to get away.

-Kalista is horrible early in the game, especially before leveling Q a little bit. Make sure to play it safe early on.
-Kalista is a duelist, but not to the point where she should be in the front line. Balance it out.
-REMEMBER TO ULT IF SOMEONE NEEDS IT~~~~~
-Your ult is on a short cooldown. If you think you can get something out of it, use it.
-You can flash while you have someone with you - ult that Leona and flash in range for her to get right in the middle of the enemy team.
-Remember to secure dragons at level ~9 for your team.
-Don't be afraid to use E with little rend stacks if you're low on health and trying to kite. The slow will help you a lot!
-Don't overestimate Kalista's damage early. You won't be able to kill someone with 400 health if you don't have a few points in Q.