
Reviewed for patch 7.3b

Favorable matchup
Reviewed for patch 7.3b
Twitch's damage builds up: Deadly Venom deals damage over time and Contaminate cashes in every stack on poisoned targets. Lucian wins in the opposite format, short trades of an ability plus a double shot, and Relentless Pursuit lets him end the trade before the venom stacks.
Every Lucian ability turns his next attack into a double shot with Lightslinger. Piercing Light plus that double shot concentrate damage into one moment, while Deadly Venom needs several Twitch attacks and Contaminate pays off more the longer the trade lasts.
Twitch has no dash anywhere in his kit. Relentless Pursuit lets you step in for the double shot and get out before he answers, and its cooldown drops every time you land Lightslinger on a champion.
Trade when Twitch walks up to last-hit and Ambush is on cooldown. If he vanishes with Ambush, don't look for the trade: back off toward your support until he reveals himself.
If you're playing Twitch, avoid the short trades Lucian picks: give up minions at range and look for fights where your venom has time to stack. Use Ambush to come in from an angle that doesn't let him dash back to his support, and save Venom Cask for after he dashes.

Favorable matchup
Reviewed for patch 7.3b
Twitch needs to be close and keep attacking for the venom to matter. Ezreal pokes from outside that range with Mystic Shot and has Arcane Shift to leave right as Twitch appears with Ambush's attack speed, which leaves his ambush with no target.
When he leaves Ambush's camouflage, Twitch gains attack speed for a while. Arcane Shift blinks you away the moment he reveals himself: if you get out of his range, he spends part of that window walking toward you instead of attacking.
Mystic Shot deals damage from range and lowers your cooldowns when it hits. Against a Twitch with no dash, every hit on a last-hit costs him health without letting him answer with attacks; Essence Flux adds damage when you detonate it.
Poke when Twitch steps up to last-hit and isn't camouflaged. If he disappears, hold Arcane Shift: it's your answer to his appearance, not a damage tool while Ambush is up.
Twitch against Ezreal has to draw out the blink before committing: come out of Ambush far enough away to force it, then go in for real while Arcane Shift is on cooldown. Dodge Mystic Shot by moving behind your minions and use Venom Cask to slow him after the blink.

Execution matters
Reviewed for patch 7.3b
Ashe slows with every attack and can stun from range with Enchanted Crystal Arrow, exactly what punishes a dashless Twitch the moment he leaves Ambush. It's situational: Ashe has no escape either, and if Twitch appears next to her unannounced, the trade is his.
Ashe's attacks slow with Frost Shot, and Volley applies that slow in a cone. Twitch has no dash: once he's revealed, every slow stops him from closing in or backing off at his own pace, and you control the distance of the trade.
Ambush ends when Twitch attacks or uses offensive abilities. Enchanted Crystal Arrow stuns the first champion it hits, for longer the further it travels: save it for the moment he reveals himself, before he chains attacks or starts Spray and Pray.
Hawkshot sends your hawk out to scout. Use it to check the river or the brush Twitch usually comes from, and to spot his support. Don't assume the hawk detects a camouflaged Twitch: its value is knowing where he isn't.
Trade while Twitch is visible and Ambush is on cooldown. If he disappears, back off next to your support and save the ultimate for when he shows up.
From Twitch's side, the key is to appear where Ashe can't aim the arrow in time: come in from a flank with no vision and don't reveal yourself at mid range. Stay out of her range while she has Volley ready and punish once she has used Enchanted Crystal Arrow; without it, her answer to your ambush is much weaker.