Please do not buy Kalista. She relies on you knowing what you're doing. And when I say that, I don't mean knowing how to play her; you have to KNOW what you are doing. Where your hands are, the lat/long on the map of you and your Oathsworn, the gender of your laning opponent based on their playstyle, where their hands are, your damage, their damage, their defense, whether your support knows all of this or not... Here's an equation for you!
Solve this, and you MIGHT JUST get a kill:
https://gyazo.com/1399c1a6f498244f28f3643b617d795e
Granted, this is just ONE kill. If you want a Penta, solve THIS:
https://gyazo.com/5ef05616841d3f56b7a8e2179c45043f
I'd recommend Azir. He's pretty fun, when he isn't eating bugs from his nest.
If you want to win, play Fiora against onyone that isnt Renekton.