Kassadin is at his weakest in the early game. Try to fight as much as possible so he falls behind.
Post level 6, if you have any CC abilities, do not use them unless his Ultimate R is down as he will use their cooldowns to play aggressive and trade with you.
Kassadin lacks wave clear in the early game. If you can keep the wave closer to your side of the map early on, it will be very difficult for him to farm or break your freeze. If he does walk up to last hit, you can look to trade with him.
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 R.
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.
Kassadin is very weak in the early game. Try to exploit this to gain a lead and prevent him from taking over the map. Giving him a rough start will reduce his scaling.
Once Kassadin has his Ultimate R, he will be able to trade more frequently and also escape danger. At this stage, his kill pressure also increases.
Kassadin is a later game monster and once he’s level 16, he will be very slippery thanks to his Ultimate R and short cooldowns. Try and end before he gets a chance to reach level 16.