Xianyun

5★

AnemoCatalystSupport

TL;DR

Xianyun exists to make falling on people a viable strategy. Her Burst lets your active character jump much higher and turns their next eight plunging attacks into serious damage, while healing the team the whole time. Her Skill is how she gets her own energy back and does a chunky Anemo plunge of her own. If nobody in your team plunges, she's a fine healer and nothing more, so only build her if you have someone to launch.

Rotation

Skill (E) and plunge with it, for the particles.
Burst (Q) to drop the lantern.
Swap to your plunge character and spend the rest of it jumping and falling on things.
Repeat. She wants only a few seconds of field time per rotation.

Abilities

Skill (E)	She leaps forward, up to three times if you keep pressing. End it with a plunge for a big Anemo hit and a guaranteed five energy particles. The particles are why you do this every rotation.
Burst (Q)	Drops a floating lantern that heals your team every couple of seconds, lets the active character jump far higher than normal, and hands out eight charges of plunge damage. Lasts about 16 seconds.
Plunging attack	Jump, then attack on the way down. This is the button her whole kit is pointing at, and it belongs to your damage dealer, not her.

The one mechanic to understand

Eight plunges, counted per landing.

The burst hands out eight charges and one plunge spends one of them, however many enemies it lands on. So you get eight buffed plunges whether you're on a boss or in a crowd. Only plunges that actually reach the ground count, so clipping something in mid-air on the way down neither spends a charge nor gets the damage. The catch with a crowd is that the extra damage only lands on one enemy per plunge, so the charges go just as far but each one pays out on a single target. The extra jump height from the lantern is there to give you time to line the plunge up.

What actually matters

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

Someone who plunges. Without one you have spent a 5-star on a healer. With one, plunging goes from a party trick to your main damage button.
Attack. Both her healing and the plunge bonus scale off her own attack stat, and nothing else in her kit competes. The bonus stops growing at around 4500 attack, which is far more than you will have, so just stack it.
Energy Recharge. Her burst is the character and it's expensive. Around 200% if she's your only Anemo, less with a second one. The plunge off her skill giving a flat five particles is what makes that reachable.

Gear

A build that works. KeqingMains lists every other option.

Weapon	Energy Recharge until her burst is reliable, then Attack. Favonius Codex is the easy pick, since the extra particles it throws out are exactly what she's short of.
Artifacts	4-piece Viridescent Venerer. Her Anemo hits swirl whatever your team is applying, and the set drops that element's resistance by 40% for everyone.
Main stats	Energy Recharge sands until she can cast every rotation, then Attack sands, Attack goblet, Attack circlet. Crit is worthless on her.

Works well with

Pyro

Hydro

Electro

Cryo

These are the elements her Anemo can swirl for the resistance shred. What she actually needs is a plunging damage dealer, and those come in most elements.

Don't bother

Crit stats. Nothing in her kit scales off them, including the healing.
Plunging with her instead of your damage dealer. Her plunge is for particles, the charges are for someone else.
Counting the eight charges. Plunge until the buff feels gone, then re-cast.
Building her at all if nobody in your roster plunges. She is a good healer, but you have cheaper ones.

Want the full theorycrafting version? Read Xianyun on KeqingMains.