
Reviewed for patch 7.3b

Favorable matchup
Reviewed for patch 7.3b
Wind Wall blocks enemy projectiles for four seconds, and Riot describes two of Hwei's spells as a projectile or missile: Devastating Fire and the Gaze of the Abyss eye. Blocking them takes away his ranged damage and his root.
Riot describes Devastating Fire as a projectile and Gaze of the Abyss as an eye that sends a homing missile. Grim Visage, Severing Bolt, Molten Fissure and Crushing Maw aren't described that way: don't count on the wall, walk out of them instead.
Hwei's mobility is Fleeting Current, a strip of speed, not a dash. Sweeping Blade through minions brings you closer without walking a straight line through his zones, and 7.3b cuts Crushing Maw's range from 7.5 to 7, shrinking the distance from which he can drag you.
Signature of the Visionary explodes when a second, different spell hits the same target. 7.3b lowers that damage and raises the Way of the Wanderer shield: arrive with full Flow and the shield absorbs part of the combo.
Go in once Hwei has spent his Torment spell or you've blocked it with Wind Wall. If he still has Torment and your wall is on cooldown, farm from range and let the trade go.
As Hwei, the rule is to wait out the wall: open with Severing Bolt or Molten Fissure and save Gaze of the Abyss for when Wind Wall is on cooldown. Place Crushing Maw between Yasuo and the minions he uses to approach, keeping its shorter 7.3b range in mind.

Favorable matchup
Reviewed for patch 7.3b
Hwei needs his target to stay in his zones long enough. Fizz goes in with Urchin Strike and, when Hwei lines up his crowd control, hops with Playful / Trickster to land outside the zone.
While Fizz is on the trident he can't be targeted, and the hop moves him somewhere else. Use it when Hwei picks Torment to land off Grim Visage's path or outside Crushing Maw's zone; recasting drops you early in a direction, with a smaller splash and no slow.
Hwei has no dashes: his escape is Fleeting Current's speed. Urchin Strike carries you through him and applies on-hit effects, so you close the gap when he has spent the current or painted it in a direction that doesn't help him.
The Chum the Waters fish attaches to the champion and reveals them; after a short delay, the shark knocks them up. On a Hwei who has already used Fleeting Current, it leaves little room to escape and sets up your engage.
Look for Hwei once he has spent his Torment spell or Fleeting Current. If he still has both, wait with Playful / Trickster available and farm with Rending Wave.
If you're Hwei against Fizz, don't spend Torment before he jumps: poke with Disaster and keep Grim Visage or Gaze of the Abyss for when he comes down from the trident. Pool of Reflection absorbs part of his burst, though it protects less in 7.3b. Paint Fleeting Current toward your tower before your health runs out.

Execution matters
Reviewed for patch 7.3b
Kassadin has tools against a zone mage: Void Stone shields him from magic damage when he casts nearby, Null Sphere silences, and Riftwalk takes him out of Crushing Maw or Spiraling Despair. Before his ultimate, Hwei punishes him.
Hwei picks a Disaster, Serenity or Torment spell before casting it, and the Signature of the Visionary burst needs a second, different spell. Null Sphere silences the first enemy hit: landing it mid-sequence delays that second spell.
Void Stone grants a shield against magic damage when you cast a spell near a visible enemy champion. Hwei's damaging abilities deal magic damage, so the shield soaks part of his answer every time you trade up close.
Riftwalk is a blink. Crushing Maw drags toward its center and Spiraling Despair grows before it explodes: the blink gets you out of both without relying on speed.
Before you have Riftwalk, farm with Nether Blade and avoid trades. With your ultimate, go in once Hwei spends his Torment spell.
Before Riftwalk, Kassadin has no way to close in: use those levels to punish him with Devastating Fire and Severing Bolt. Later, keep Gaze of the Abyss, whose missile seeks the nearest champion, for when he appears on top of you. His shield absorbs magic damage: don't empty your palette while it's up.