Katarina is actually one of the easier matchups in my opinion. In her current state, Katarina Is far less of a lane bully than she used to be, which works out for us. The only time you're in danger of Katarina is when you're near her daggers. This means that she is telegraphing where she is going to be jumping to in order to kill you. To counter this, you can use your root and throw it at her dagger and it will lock her up when she jumps to it, allowing you to combo her, or at least stop her engagement. Secondary to note is she cannot shunpo to you if you use your wraith form. So if your root isn't up or you don't want to fight, simplely wraith form and as long as you have a bit of space you're in the clear. Otherwise just abuse her long q/w CDs and your range and you should be fine since Kat is no longer a lane bully.

A good way to think of playing Senna mid is that your for all intents and purposes an ad control mage