Well I've mastered Katarina long time ago, she is basically the champ I play once I know I want to win a-game, I may say that once you know how to play her she is hardly countered, for those who say "One cc and she dies", well its all about your brain.exe and how you initiate into fight and calculate every guy cooldown.

Feel free to ask anything, I'm new to the guide-posting and all so I might try to make this guide more in-depth though Katarina is all about practice and practice.

This is still experimental, but the idea is to literally boost your output damage
to the max with less sustain as Katarina is a burst assassin.

Basically rushing for AP runes only, with some armor as you play vs AD team only,
I find scaling runes better for your damage output, and armor for some sustain.