
Reviewed for patch 7.3b

Favorable matchup
Reviewed for patch 7.3b
Syndra relies on concentrated magic burst. Galio blunts it with Shield of Durand, which reduces magic damage while you charge it, and with his passive shield against magic damage; then he answers with Justice Punch, a charge that knocks up the first champion.
While you hold Shield of Durand, Galio reduces magic damage more than physical, and every so often he gains a passive shield against magic damage when he takes damage. Charge it when Syndra lines up Unleashed Power or Scatter the Weak, not during her loose poke.
Releasing Shield of Durand taunts nearby enemy champions. If Syndra steps in to place spheres and push them, the taunt makes her walk toward you instead of keeping the distance she needs.
Hero's Entrance gives allies in the area a shield against magic damage and lands after a short delay, knocking enemies up. If Syndra roams to pick off another lane, arriving with that shield takes value away from her burst.
Answer when Syndra spends Scatter the Weak or has few Dark Spheres near you: with no sphere in line she can't stun you. If she has spheres around her and the push ready, charge the shield instead of engaging.
When you play Syndra, punish Galio when he uses the shield on the wave: poke with Dark Sphere and Force of Will while it's on cooldown. Don't spend Unleashed Power while he's charging the mitigation. Keep Scatter the Weak to answer his Justice Punch or his landing.

Favorable matchup
Reviewed for patch 7.3b
Syndra's stun requires lining up sphere, push and target. Zed breaks that alignment by swapping places with Living Shadow, and Death Mark makes him untargetable during the dash toward her.
Scatter the Weak stuns when a pushed Dark Sphere hits the target. Recasting Living Shadow swaps your position with your shadow's and, with it, the line Syndra had worked out for the push.
Unleashed Power is cast on a champion. During Death Mark's dash, Zed is untargetable; go in before Syndra casts it and she can't pick you during the dash; if it's already cast, don't count on dodging it this way. The shadow left behind lets you return.
Zed and his shadows throw Razor Shuriken. From a forward shadow you punish Syndra without bringing your body into the sphere zone, and you regain energy when two shurikens hit the same enemy.
Go in when Syndra spends Scatter the Weak or has few spheres around her. If the push is ready, poke from the shadow and keep the swap.
Against Zed, Syndra wins with patience: don't fire Scatter the Weak at the first shadow you see, but when Zed swaps or finishes the Death Mark dash. Put spheres between yourself and his shadow. Save Unleashed Power for when he can no longer turn untargetable.

Execution matters
Reviewed for patch 7.3b
Syndra packs her damage into a few seconds. Chronobreak returns Ekko to where he was a few seconds earlier, makes him untargetable while he rewinds and heals him based on the damage taken in that window. It only works if Ekko can cast it: a well-timed stun denies it.
Chronobreak heals based on damage taken over the last few seconds and damages enemies near the arrival point. If Syndra spends Unleashed Power on you and doesn't finish you, you come back with health restored and her ultimate on cooldown.
Parallel Convergence creates an anomaly that, after a few seconds, slows and stuns if Ekko enters it. Placed where Syndra plants herself to push spheres, it forces her to move or accept the stun.
Phase Dive moves Ekko and his next attack gains range and teleports him to the target. It's how you reach Syndra once she has spent Scatter the Weak.
Go in after Syndra uses Scatter the Weak. With Chronobreak available you can take her ultimate; without it, don't commit while she has spheres around her.
Against Ekko, the order of your spells decides the trade: keep Scatter the Weak for his Phase Dive and cast Unleashed Power once he has used Chronobreak or is stunned. Don't wait inside the Parallel Convergence zone for him to step in. Ekko returns to where he was a few seconds ago: move away from that spot.