Against enemy mages, Null Sphere is an effective harassment tool that is unblockable by creep waves. The shield can be used to absorb spells sent in retaliation.
Null Sphere is expensive at its first tier. Its mana cost is lower per level in terms of damage dealt as points are spent on it; max it first only if you plan to poke the enemy laner and not to farm with it.
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.
Champion 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.
As an assassin, Kassadin excels at ganking. Also, he is best at focusing lone, fragile or injured targets. With all of this in mind, Kassadin performs best entering in-progress team fights, either slaying a carry quickly and jumping out, or moving in once an enemy carry has been slain and the enemy team is beginning to disperse or retreat, to pick off injured champions.
Unless it is urgent, when traveling with Riftwalk wait until just before your last Riftwalk counter has expired before casting it again as stacking Riftwalk can deplete your mana very quickly.
Riftwalk can be used to jump over walls, and essentially acts like the summoner spell Flash.
On Twisted Treeline, you can Riftwalk from the wall behind your nexus to the nearby Shadow Altar or vice-versa. This is extremely useful if you need to escape after locking an altar.
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 can be built as a high-mobility mage or as an ambush assassin, and his prime statistics are ability power and mana (as both his Riftwalk damage and Nether Blade mana restore scale in mana).
In a similar vein like Ryze, Kassadin can be effectively built tanky since most mana items often grant defensive stats like health and armor. In fact, it's actually beneficial to build a bit tanky on Kassadin since he needs to get close to eliminate enemies.
Since building AP and having high AD, along with gap closers and short cooldowns, Lich Bane is especially usefully in most offensive builds. It also provides him with more movement speed and mana with his bonus damage.
Iceborn Gauntlet is a popular and arguably better choice on Kassadin for several reasons: It provides a small amount of AP, but more importantly, it provides a huge amount of mana. Additionally, Iceborn Gauntlet's Spellblade passive scales off of base champion AD, and Kassadin happens to have the 3rd highest base AD at level 18. The extra armor is also invaluable for tower diving and for being in the middle of a fight.
With his Nether Blade passive and active, it is possible to build Kassadin as Magical DPS with Nashor's Tooth or Wit's End, though due to the rarity of the build and lack of proper testing, the effectiveness of this build is questionable.
If attempting such a build, Arcane Blade [[Arcane Blade|]] and high AP are necessary along with some tankiness.
While Manamune seems to be an interesting item to build for Muramana, the passive will not proc from Force Pulse or Riftwalk, making this a gimmicky offensive item for AD or hybrid builds. An Archangel's Staff is a better option since it not only provides ability power scaled based on mana, but he also gain a defensive shield that expends a percentage of mana once it's upgraded to a Seraph's Embrace, which synergizes well with Null Sphere. However, like Ryze, it is conceivable to build two Tear of the Goddess for both items at the expense of stacking both Tears longer.
Kassadin is a fearsome anti-mage due to Void Stone and Null Sphere. Since he specializes in countering AP champions, an AD mid is a good counter in laning phase.
Kassadin mostly deals magic damage; therefore, stacking magic resistance will mitigate his damage.
Buying magic penetration to face Kassadin is not recommended, as his passive doesn't give him magic resistance; the way Void Stone works is by reducing 15% of the Magic Damage taken after applying resistances, so magic penetration will only affect him partially.
Force Pulse can only be activated if he has enough stacks. Engaging him when he has no stacks can help kill him faster.
In the laning phase, especially on a solo lane, it can be very hard for Kassadin to keep stacks on his Force Pulse, so casting spells sparingly can greatly reduce his damage output. Manually detonating a two-part spell such as Lucent Singularity or Death's Caress adds an extra charge to Force Pulse, so it may be beneficial to let these spells expire on their own.
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.
Be aware that Null Sphere's shield is immediate and does not depend on the projectile landing.
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 him or at least taking him down with you.
Because of the mobility offered by Riftwalk, it is crucial that your jungler ganks Kassadin before he reaches Level 6. Chasing Kassadin when he has Riftwalk is not practical.
Pantheon is known for dominating Kassadin early game due to his constant Spear Shot pressure and killing potential. Also, Aegis of Zeonia can be utilized to lock down Kassadin and assist ganks from the jungler.