Hello im Nvile and probably the best Kassadin in the world. Im gonna show you my secret to be pro kassadin

It's very important to watch your opponents early moves, theese moves are very important as you will be able to see what playing style he/she got.

Build
I usually start out buying a crystalline flask and 3 health pots.
This helps great early sustain. I recommend farming and avoid playing aggresive pre6.
Kassadin got great burst but you have to remember using it right! Fx. try to hit the enemy with your (R) ultimate, when you choose to land a burst, as it deals insane dmg.