Try to catch enemy carries alone to burst them with Icathian Rain.
Work with your teammates to set up your Ultimate and optimize damage with your passive.
Make sure to buy items that will evolve at least 1 or 2 of your spells.
Kai'Sa is very good at picking off isolated enemies. Stick together against her.
Kai'Sa is very vulnerable to getting outranged by mages and long range carries.
Make sure to place wards in your blind spots so you can see Kai'Sa coming.