
Reviewed for patch 7.3d

Favorable matchup
Reviewed for patch 7.3d
Against Shen, Gwen hits where his kit defends least. Thousand Cuts deals damage based on the target's health, Snip Snip! applies it on every center snip and Spirit's Refuge only blocks attacks, not abilities. The more health Shen stacks, the more every snip is worth.
Thousand Cuts adds magic damage based on the target's health to your attacks and heals you for part of that damage against champions. Shen is a tank who adds health with his items, and that bonus damage grows with it instead of falling off.
Spirit's Refuge creates a zone that blocks enemy attacks. Snip Snip! is an ability: it snips in a cone up to 6 times, deals true damage in the center and applies your passive on every snip to whoever is there. Even with the zone protecting Shen, the snips still land.
Hallowed Mist protects you from enemies outside it: only those who enter the mist can target you. Shen needs to get close to hit with Twilight Assault; inside your mist, the duel is played on your terms.
From level 1, look for short trades with attacks and a charged Snip Snip! when Shen walks up to last-hit. The window closes when he has Twilight Assault ready and you don't have the mist.
Playing Shen, don't give Gwen long trades: every attack she lands charges her snips. Use Shadow Dash to cut off her engage instead of opening with it, and step into Hallowed Mist if you want to keep hitting her. Spirit's Refuge protects you from attacks, not from Snip Snip!: get out of the center of the cone.

Favorable matchup
Reviewed for patch 7.3d
Shen wants to reach the teamfight phase with his ultimate ready to help another lane. Jayce makes him pay for that wait: he pokes with Shock Blast in Cannon Stance, pushes the wave and turns every Stand United into minions and tower damage for his team.
In Cannon Stance, Shock Blast fires an orb that detonates on hitting an enemy or at the end of its path and damages everyone caught. Fired through Acceleration Gate, it gains speed, range and damage. Shen has to come close to hit; you don't.
Mercury Cannon is the transformation between hammer and cannon. In Hammer Stance, Thundering Blow knocks Shen back and To the Skies! leaps onto him: if he walks up to use Twilight Assault, swap stances and push him away.
Stand United shields an ally and, soon after, teleports Shen to them. Every time he uses it, his lane is left with nobody in it: with the wave pushed, that time belongs to your minions and the tower.
Every wave, poke with ranged attacks and Shock Blast. The key window comes when Shen uses Stand United: push right away, because his lane is empty.
If you're Shen, don't give away the wave to use Stand United: do it when Jayce has recalled or the wave is near your tower. In lane, recall your blade through him so Twilight Assault comes out empowered, and go in with Shadow Dash once he has spent Thundering Blow.

Execution matters
Reviewed for patch 7.3d
Mordekaiser doesn't beat Shen on lane tempo, but in one specific moment: Realm of Death drags him to another dimension and steals part of his stats. He needs the ultimate and the health for that duel; without it, Shen sets the pace.
Realm of Death drags Shen with you to a different dimension and steals part of his core stats; if you take him down, you keep them until he respawns. The idea is to pull him out of the rest of the fight for that time.
Indestructible stores the damage you deal and take to create a shield you can later turn into health. Twilight Assault hits based on max health: with the shield up, that damage stays on it.
Death's Grasp pulls in every enemy in an area. If Shen tries to get away with Shadow Dash before your ultimate lands, the pull brings him back into Obliterate range.
Play the early levels without losing health: last-hit with Obliterate on minions. With Realm of Death up and Shen without a big health lead, that's your window; outside it, the long trade isn't worth it.
As Shen, the rule is never to enter the realm behind: if Mordekaiser has his ultimate, keep your health high and don't stand close when he uses Death's Grasp. If he drags you in, use Shadow Dash to taunt him and buy time with Ki Barrier, and play the duel with Twilight Assault recalled through him.