Zhongli

5★

GeoPolearmShielder

TL;DR

Hold Zhongli's Skill and you get a shield so far above what the game throws at you that you can mostly stop paying attention to your health bar. It also drops enemy resistance to every element and to physical damage by 20% while it's up, which quietly buffs your whole team. He costs you one team slot and about eight seconds a minute, and in exchange difficulty goes down and damage goes up. Stack Max HP, that's the build.

Rotation

Hold Skill (E) for the shield.
Swap to your team and play the fight.
Come back and hold E again every 20 seconds or so, before the shield times out.
Use Burst (Q) only when you actually want things frozen in place. It's optional.

Abilities

Skill (E)	Hold it. That gives you the shield, which lasts 20 seconds and absorbs far more than its listed number suggests, plus a stone pillar. Tapping gives you the pillar and no shield, which is not what you came for.
Burst (Q)	A meteor lands, does respectable damage and turns most normal enemies to stone for a few seconds. The cast animation is long, so it costs your damage dealer field time.

The one mechanic to understand

The shield does two jobs.

Everyone knows about the first one. The second is that while the shield is up it lowers nearby enemies' resistance to every element and to physical damage by 20%, which is a straight damage increase for whoever you are actually playing. It does not stack with a second copy of itself, but it stacks with everything else. This is why Zhongli shows up in teams that were never in any danger of dying, and it's the reason he's worth a slot even on easy content.

What actually matters

The two or three real decisions. Everything else is noise.

Hold, don't tap. The single most common mistake with him. A quick press gives you a pillar and no shield.
Max HP is the only stat that matters. The shield's size comes entirely from his HP. Not attack, not defence, not his talent levels beyond the skill.
Don't go hunting shield strength. It isn't an artifact stat in the first place, and the shield is already far bigger than it needs to be. If you want more out of it, level his skill talent, which is cheap and does the job.

Gear

A build that works. KeqingMains lists every other option.

Weapon	Max HP. Black Tassel is a 3-star polearm with the highest HP% main stat in the game, and on a character who deals no damage it beats most 5-stars. You almost certainly already own several.
Artifacts	4-piece Tenacity of the Millelith. His shield keeps enemies near him hit constantly, so the set's 20% attack buff for the party stays up basically all the time.
Main stats	HP sands, HP goblet, HP circlet. Ignore crit, ignore attack, ignore Elemental Mastery.

Works well with

Geo

Pyro

Electro

The shield and the resistance drop help literally any team, so treat these as a mild preference rather than a requirement. Geo is listed because a second Geo character gives the pair a flat attack bonus.

Don't bother

His burst, most of the time. It's slow and your damage dealer wants those seconds.
Energy Recharge, unless you have decided you want the burst every rotation. Skipping the burst skips the problem.
Building him for damage. People have tried. It's a novelty.
The stone pillar. It does a small amount of damage and it's mostly just something the shield comes attached to.

Want the full theorycrafting version? Read Zhongli on KeqingMains.