A risky way to deal a lot of burst damage to enemies is to constantly cast Riftwalk at the summoner pool. This will stack the damage on Riftwalk while the summoner pool regenerates your mana. Using this method with summoner spell Teleport can inflict devastating damage.
Kassadin is fairly easy to kill if he can't Riftwalk away. When engaging the enemy, consider whether it'd be better to Riftwalk in, cast your spells, and walk out, or to walk in, cast your spells, and Riftwalk out.
You shouldn't Riftwalk in unless you are certain you can either kill your opponent or that they won't retaliate much. Casters will usually not be able to react quickly enough before the Null Sphere lands, but AD dps are normally unaffected by your silence.
Riftwalking into a team fight will probably draw attention to you, something you want to avoid. As such, walking into a team fight and teleporting out can help you avoid retaliation.
Unless it is urgent, when traveling with Riftwalk wait until your last Riftwalk counter has expired before using it again as Riftwalk can deplete your mana very quickly.
Kassadin's abilities have high ability power ratios and low cooldowns, so a high amount of AP enables him to deal immense damage in short time intervals.
The Riftwalk, Null Sphere, and Force Pulse combo is especially powerful against casters, as it silences and heavily damages them before they can react to it. Make sure you have at least 4 charges in Force Pulse before doing this combo, as Riftwalk and Null Sphere will let you gain the remaining 2 charges needed to cast it.
Riftwalk can be used to jump over walls, and essentially acts like the summoner spell Flash.
As an assassin, Kassadin excels at ganking. Also, he is best at focusing alone or weak targets, or even targets like casters or low-MR enemies. With all of this in mind, Kassadin performs best entering in-progress team fights, once a carry has been slain and the enemy team is beginning to disperse or retreat, to pick off injured champions.
With his Void Stone and his Nether Blade active, it is possible to build Kassadin as Magical DPS, though due to the rarity of the build and lack of proper testing, the effectiveness of this build is questionable.
Kassadin has trouble killing minions waves quickly in the early game, as Force Pulse is not always available, leaving him with only melee attacks and the long-cooldown Null Sphere. Playing a champion with strong AoE spells can allow you to push your minions to Kassadin's turret, which will force him to either lose gold income when the turret kills your minions, or stay in melee range of your minions, leaving him vulnerable to your ranged spells and attacks from outside the turret's range.
Predicting 卡萨丁的Riftwalk movement can be greatly effective in countering him, as his harass combo involves mostly Riftwalk into range, and dropping a Null Sphere.
The travel time of Null Sphere is noticeably slow - if you have your own crowd control that can be cast onto Kassadin before it lands and after he uses Riftwalk to approach you, you can punish him and favourably trade.
As a melee assassin, Kassadin must close the gap to begin fighting, and, more importantly, must initiate the fight to do all of his practical damage. With this in mind, being able to close in before he can Riftwalk or even applying your crowd control first will considerably buff your chances of slaying or at least defeating him.
Do not use Flash to get away from Kassadin because of his Riftwalk can still be used to teleport to a new area.
Warwick is a good counter for multiple reasons. Firstly, his basic magic resistance makes him suitably less squishy than other fighters versing Kassadin. Secondly, his life steal assures that his health is buffed throughout the fight. Finally, his ult, Infinite DuressInfinite Duress, prevents Kassadin from using Riftwalk to initiate the fight, as it suppresses him and places Warwick right on top of him.
Mordekaiser is another excellent counter, for having even higher base magic resistance than Warwick, and that the shield he gains from ability use is very capable of soaking damage from Kassadin's spells.