Mona

5★

HydroCatalystSupport

TL;DR

Mona's Burst drops a bubble over the enemies. The next hit that lands pops it and leaves a mark on them that makes everything they take hurt more for about 5 seconds. Her Skill puts down a decoy that soaks aggro and keeps them wet. So you press both, swap out, and your damage dealer gets a short window where their numbers are inflated.

Rotation

Skill (E) to drop the decoy.
Burst (Q) on top of the enemies.
Swap to your damage dealer immediately and spend the next 5 seconds.
Come back when the burst is up again, about every 15 seconds.

Abilities

Skill (E)	Drops a decoy that taunts enemies, ticks Hydro damage into them and explodes when it expires. Hold it to backdash first. 12 second cooldown, and only one can be out at a time.
Burst (Q)	Costs 60 Energy, back in about 15 seconds. Bubbles the enemies, soaks them and holds the small ones still. The first hit they take pops the bubble and leaves the mark, which lasts about 5 seconds.

The one mechanic to understand

The five seconds after the pop.

The bubble itself does nothing. What you want is the mark it leaves when something breaks it, and that mark only lasts about 5 seconds. The hit that pops the bubble gets the bonus too, so there's no wasted swing and no reason to be clever about it. What there's no room for is faffing about: burst, swap, and have your damage dealer already swinging. Cast it and then spend three seconds walking into position and you've thrown most of it away.

What actually matters

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

Energy Recharge is never a wasted stat on her. One of her passives turns Energy Recharge into Hydro damage bonus, so the stat she needs for burst uptime is also a damage stat. Around 180% is comfortable.
Her decoy is real Hydro application. It keeps enemies wet the whole time it's out, so she can be the Hydro in a team without ever coming back on the field.
Line the mark up with your biggest hit. If your damage dealer has a slow wind-up or a burst on a long cooldown, that's what the window is for.

Gear

A build that works. KeqingMains lists every other option.

Weapon	Energy Recharge. Favonius Codex is the straightforward pick. Thrilling Tales of Dragon Slayers is worth considering instead: it's a 3-star you almost certainly already own, and swapping off her hands the next character a large attack buff, which is what she's there for anyway.
Artifacts	4-piece Noblesse Oblige. She casts her burst every rotation regardless, and the set turns that into 20% attack for the whole party.
Main stats	Energy Recharge sands, Hydro goblet, Crit circlet. If the burst isn't coming back every rotation, take the crit off the circlet and put more Energy Recharge on.

Works well with

Cryo

Electro

Pyro

Anything that reacts with Hydro. Frozen enemies are the friendliest case, since they sit in the bubble and can't walk out of the mark.

Don't bother

Building her to deal damage herself. She's on the field for two button presses.
Her water sprint in a fight. It's a lovely way to cross a lake and a good way to be out of position.
Bursting on something that's about to die anyway.
Waiting for a big attack to pop the bubble. Any hit does it, and the clock is the enemy.

Want the full theorycrafting version? Read Mona on KeqingMains.