Diona

4★

CryoBowShielder

TL;DR

Hold Diona's Skill for a shield, press her Burst for a puddle that heals whoever stands in it, then get out of the way. She does both jobs at once, which is the whole appeal: one slot covers your shielding and your healing and throws some Cryo around while it's there.

Rotation

Hold Skill (E) until she dashes back and fires. That's the big shield.
Press Burst (Q) on top of the enemies, since it has to cover your carry too.
Swap out. The puddle heals and chips away for about 12 seconds.
Come back every 20 seconds or so for the next burst, and re-shield whenever hers has gone.

Abilities

Skill (E)	Fires icy paws that shield whoever is on the field when they connect. Tap for two paws on a 6 second cooldown, hold for five on a 15 second one. The shield scales off her Max HP and it's very good against Cryo specifically.
Burst (Q)	Throws a drink that leaves a mist on the ground for about 12 seconds. It heals anyone inside and does slow Cryo damage to anything inside. Costs 80 Energy, 20 second cooldown. Enemies caught in it also lose 10% attack.

The one mechanic to understand

Hold the Skill, don't tap it

Each paw that lands adds about 2.4 seconds to the shield, and the held version fires five instead of two while also making the shield roughly 75% stronger. So holding gets you something like a 12 second shield and tapping gets you 5 seconds of a much weaker one. The hold has a longer cooldown, 15 seconds against 6, but the shield lasts long enough that you don't need it back sooner. Tap only when you want the cooldown back fast for energy particles, or when there is nothing to shield against and you just want Cryo on the enemy.

One thing worth knowing: the shield puts Cryo on your own character for a moment as it forms. That's fine or even useful most of the time, but it will react with a carry who was carrying their own element, so if your Pyro character's numbers wobble right after a shield, that's why.

What actually matters

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

Max HP is both of her jobs. The shield scales off it and so does the healing. HP% on everything, and attack does nothing for her.
Energy Recharge, and a fair bit of it. 80 Energy on a 20 second cooldown is a real cost for a character who only presses two buttons. Somewhere around 160% keeps the burst coming every rotation, and a Sacrificial or Favonius bow does a lot of that work for you.
Put the puddle where the fight is. It doesn't move, and a healing puddle nobody is standing in is just a bit of Cryo damage.

Gear

A build that works. KeqingMains lists every other option.

Weapon	Energy Recharge. Favonius Warbow if you have it, since the extra particles help her afford that 80 cost. Sacrificial Bow is the other easy one, and it sometimes hands her Skill straight back.
Artifacts	4-piece Noblesse Oblige for the party attack bonus. Swap to 4-piece Deepwood Memories if you're in a Dendro team and nobody else is holding it, since the resistance shred is worth more there than her own numbers.
Main stats	Energy Recharge or HP% sands, HP% goblet, HP% or Healing Bonus circlet. Skip crit entirely unless you're running Favonius and need the crit to trigger it.

Works well with

Hydro

Cryo

A shield and a heal fit anywhere. Those two are just what her Cryo does something with: freezing on Hydro, and a second Cryo character turns on the resonance bonus against frozen enemies.

Don't bother

Shooting things with her. Her bow damage is nothing and she has no reason to hold the field.
Building attack or crit. Neither one touches the shield or the healing.
Aiming carefully with the hold. She dashes backwards as part of it, so aim roughly at the enemy and let the paws find them.
Expecting the C6 Elemental Mastery bonus without the constellation. It's a nice trick in reaction teams but it doesn't exist below C6.

Want the full theorycrafting version? Read Diona on KeqingMains.