Sion vs Malzahar

Last-hit minions and monsters for Soul Furnace (W): each kill permanently gives you max health, more on champions, large minions and large monsters.

Slow your target with Roar of the Slayer (E) before charging Decimating Smash (Q): they're less likely to get out of the area.

Charge Decimating Smash (Q) for at least 1 second: below that, the hit only slows, and beyond it, the knock-up and stun last longer the more you charge.

After you die, hit the nearest champion: during Glory in Death (passive), your autos are much faster, heal you and deal damage based on their max health.

Cast Unstoppable Onslaught (R) from far away to engage or roam: unstoppable for 8 seconds, you stun longer and hit harder after a long charge.

Get out of the Call of the Void (Q) zone quickly: its two portals fire after a short delay, and their projectiles silence you.

If you have CC, hit him first with a cheap attack, then wait a moment: Void Shift (passive) still ignores CC right after the first damage.

Stay away from minions hit by Malefic Visions (E): if they die, the visions jump to the nearest enemy, and his Voidlings follow them.

Take your trades before he hits level 6: after that, Nether Grasp (R) can lock you down from range while his team hits you.

When he casts Nether Grasp (R) on an ally, focus him: he channels in place, and any CC interrupts him and frees your ally.