Lucian vs Sivir

Weave an auto between each spell: Lightslinger (passive) turns it into a double shot, and each shot that hits lowers the cooldown of Relentless Pursuit (E).

Attack right after an ally's heal or shield, or a nearby enemy getting immobilized: Lightslinger (passive) then empowers your next 2 autos.

Cast The Culling (R) on a target your team has immobilized: each shot hits the first enemy in its path, and you can use Relentless Pursuit (E) while firing.

Look for trades and kills early on: Lucian is strongest early, and his impact drops once the other marksmen have their items.

Dodge Boomerang Blade (Q), then its return: it damages on the way out and on the way back, and it costs Sivir a lot of mana.

If you have CC, cast a cheap spell first, or fake a step forward: Spell Shield (E) only blocks one ability, and heals her if it blocks.

Back off when she casts On The Hunt (R): her whole nearby team gains move speed to engage on yours.

Don't leave her alone on a lane for too long: with Ricochet (W), her autos bounce between minions, and your wave, then your tower, fall fast.