Against kassadin, Kassadin will be split pushing during the mid-game so he can get to his level 16 power spike and then try to flank in team fights with his Ultimate k. 

If you see that Kassadin is split pushing, you could try and start a team fight with the numbers advantage as he will be unable to join the fight quickly.

Avoid stalling the game out for too long as Kassadin is a late-game monster. Try to end the game as quickly as you can. Against kassadin, Kassadin will be divided by pushing in the middle of the game so that he can reach his level 16 power peak and then try to flank in the team fights with his Ultimate k. If you see that Kassadin is divided by pushing, you can try to start a team fight with the advantage of numbers because he won't be able to join the fight quickly.