
Reviewed for patch 7.3d

Favorable matchup
Reviewed for patch 7.3d
Ashe fights from range and has no dashes. Samira gets on top of her with Wild Rush and uses Blade Whirl to destroy missiles that enter her area, Enchanted Crystal Arrow included. 7.3d trims both: Ashe loses damage per attack, Samira loses two ratios.
Blade Whirl damages around Samira and destroys enemy missiles that enter the area. Enchanted Crystal Arrow is a missile of ice: if you see it coming, the whirl wipes it out before the stun. Volley's arrows are shots too: the ones that enter the area during the whirl don't get through either.
Wild Rush dashes through an enemy, and Ashe's kit has no dash at all. Once you're on her, her answer is to slow you and keep shooting, and after the 7.3d Frost Shot adjustment those shots deal a bit less damage.
Samira's attacks against an immobilized enemy dash her into range. If your support stuns or roots Ashe, you get there without spending Wild Rush and keep it to chase or to leave, and it resets if you get the takedown.
This lane is decided up close: look for the trade when Ashe has spent Volley or the arrow, or when your support locks her down. At range, with her slows running, the long trade doesn't suit you.
If you're Ashe, play the lane at the edge of your attack range and don't fire the arrow while Samira has Blade Whirl. Slow her with Frost Shot before she uses Wild Rush and back toward your tower if her support steps up. Hawkshot tells you whether the enemy jungler is coming to help her.

Favorable matchup
Reviewed for patch 7.3d
Against Ashe, Lucian plays short trades: one ability, the Lightslinger double shot and out with Relentless Pursuit before the slows catch him. Ashe wants the trade to drag on, and since 7.3d each of her attacks hits a little softer.
After every ability, Lucian's next attack fires twice thanks to Lightslinger. Piercing Light or Ardent Blaze followed by those shots gives a short burst that doesn't require staying around to take attacks.
Relentless Pursuit is a short dash whose cooldown drops when Lightslinger hits, more so against champions. Ashe has no dash: you choose the distance of every trade, and you can also step out of Enchanted Crystal Arrow's path.
Frost Shot slows on every attack, which punishes anyone who stays in range. If you go in, shoot and leave, Ashe gets few attacks to build an edge; since 7.3d, her bonus damage per attack is also lower.
From level 2, with Piercing Light and Relentless Pursuit, look for short trades on every wave. Don't stay to fight at range: once the slow lands, get out.
As Ashe, punish Lucian when he spends Relentless Pursuit: without it, your slow keeps him in your range. Use Volley when he walks up to last-hit and save the arrow for when he comes in, not for when he's already leaving. If your support has crowd control, that's the moment for it.

Execution matters
Reviewed for patch 7.3d
Against Ashe, Sivir aims to cancel her most important play, not to beat her on attacks. Spell Shield blocks one enemy ability, and the one that opens Ashe's fights is the arrow. Everything runs through the shield: once it's spent, Ashe is in charge again.
Spell Shield blocks an enemy ability cast on Sivir and, if it does, heals her and gives her speed. Enchanted Crystal Arrow stuns longer the farther it travels: if you see it coming from far away, you have time to activate the shield and take away Ashe's opener.
Fleet of Foot gives you speed when you hit a champion, and On The Hunt speeds up your team. Against Frost Shot's constant slows, that speed is what lets your team reach Ashe or get out of her range. 7.3d leaves those slows alone and only cuts her damage per attack.
Ricochet makes your attacks bounce between nearby enemies, and Boomerang Blade deals damage on the way out and back. Pushing fast lets you move to the river before Ashe when your jungler contests scuttle or dragon.
Play the lane with Spell Shield available and look for fights once Ashe no longer has the arrow. If she uses it on another ally or misses it, your team has a window to go in with On The Hunt.
Playing Ashe, the rule against Sivir is never to fire the arrow at someone with the shield ready: aim it at her support or wait until she spends it on Volley. Keep your slow on her so Fleet of Foot doesn't carry her out of range. If she activates On The Hunt, back off: her team will arrive sooner than it looks.