As Kassadin you are a bit squishy early, so be carfull and just harass and or last shot minions with your Q. When in battle, I would suggest using R to get in the fight, then use E to deal area damage, and then finish off who ever you want with Q. If thats not enough, you can chase down your victum with your ulti and slow with E if availible, then kill with Q again. Kassadin is a great ganker since he he can ulti through walls and start fights by a serprise entrance with his ulti.

The order of my items will help you out early game and make you a durable killing machine late game. The last 2 items can be switched depending on who you are facing and if you need to kill faster or tank a bit more.

This spell will make Kassadin an even better chaser and help out in team fights as the weaker ones will have a harder time excaping with an exhaust spell on them.

Now I know what your thinking, omg heal on Kassadin what a noob. But you have no idea how much it help you early and late game. There are so many builds that say never put heal on Kassadin, and at first I listend to them, but then I was tired of being ganked and or ignited early game. So I tried it out one game, and now I use it all the time. It allows for easy get aways with your ulti, and it helps out your team in team fights aswell. I saved my self from people who want to tower dive to get me, and when I pop my heal, it makes them in range of the turret that much longer for you to get the kill, or die and the turret finishes them off for you.

Allows Kassadin to destroy squishy champions in a matter of seconds
Better chance of getting out of tough situation (like getting ganked)
Deal out a lot of damage mid and late game
Able to harass efficiently
Makes Kassadin one of the best tower divers in the game (aside from tryndameres endless rage)

Wont deal a lot of damage early game
Prone to ganks early game
May have to recall a lot early game