Kassadin vs Locke

Against a mage, take your trades with Null Sphere (Q): it gives you a magic damage shield for 1.5 seconds and interrupts channels.

Restore mana with Nether Blade (W): your next auto restores part of your missing mana, more on a champion.

Farm safely early on: your power comes from ranks in Riftwalk (R), whose cooldown drops from 5 to 2 seconds.

Chain Riftwalk (R) in teamfights while watching your mana: each quick recast adds damage, but doubles its mana cost.

Back instead of staying in lane at low health: Silver Stake (passive) adds damage to his autos that grows with your missing health.

Trade after Ashen Pursuit (E): it's his only instant movement, and its 10-second cooldown only resets if he gets a takedown.

During Soul Ignition (W), only hit him if you can kill him: at the end, he heals back part of the last damage taken, yours included.

Marked by Purgatory (R), stay above its threshold: if you have a heal or a shield, use it, because he seals marked enemies who drop below it.

If the Purgatory (R) artifact lands, hold the area where it fell: by picking it up, Locke raises his execute threshold for each sealed champion, for the rest of the game.