What to do about Katarina
There are too many games in which a whole team is losing lane except the Katarina on their team. Then the Katarina magically makes the entire opposing teams dissapear (Last time I checked Assassin meant someone who takes out people individualy not explode like a nuke and kill everything). My suggestions to Balance Katarina is to: Make her use mana; /or make her Sinister steal (W) range 100 instead of 375 /or make her passive that she is entirely cool-down based she doesn't use mana that is a good enough passive as it is.(the 15 seconds off her cooldowns is bogus even if she does have to kill/assist within a 3 second threshold she can instakill them with ease even with magic resist).
I am hoping you found my ideas somewhat sane. But untill then any game with a Katarina is a stomp unless you have a Katarina on your team.(53.85% winrate based off studies by http://loldb.gameguyz.com/statistics/winRate)
is not unbeatable. Hell press her hard with good harass and CC early game. Use ganks if you have to, and then Katarina gets hard pressed to keep up. She has literally ZERO sustain, and practically no durability. She gets focused, she dies quickly. Basically a glass cannon that has to be used like a Bowie Knife to get things done. If you can't exploit that, then it's all your fault for poor strategy, not because Kat is OP.
along with 
(who also hard counters Kat in Mid big time played right)... built right can Jungle, Lane Top, or Lane Mid. 
like it's an all you can eat buffet.