what kassadin combo should i use?

Maybe you want to stack up some riftwalks first for additional damage, but otherwise there is no specific combo to pump up your damage. You can't 100-0 carries anymore unless super fed or very late in the game :(

Best way to use a kassadin combo is finding a different champion to play.

He's beyond poop now.

That is because he doesn't do burst damage anymore. Do you have Luden's Echo and Lich Bane in your build? unless you do, he just doesn't actually have much of any AP ratios to work with (or good base damage either). Cassation traded a lot to keep his mobility, and most of that came from his CC and damage.

The ways of combo do not affect the dmg output. But they affect how fast you complete your combo, which result in less time to react for opponent. The fastest combo of Kass is Q+E+R+W. Cast Q and E before you jump in with your R and W. Higher level is Q+E+R+Auto Attack+W. W will reset your AA, and you can do more dmg.