Diona

Diona is a bow user with a Cryo element. While her normal attacks are nothing to write home about, her kit can be quite useful as a support. Her shield strength scales off of her max HP, and the duration is based on tap or hold version of her elemental skill. Her elemental burst deals damage and heals you every 2 seconds for 12 seconds. Since it also applies Cryo element every 2 seconds, it's useful for elemental reaction with your team. Her main downside is that her elemental skill has quite a lengthy cooldown, and elemental burst is costly at 80 energy to use.

The best weapon to use for her is the Sacrificial Bow. It adds a lot of value to her since doing the hold version has quite a long cooldown, but it gives her about 4 elemental particles, and another 4 if it resets again. The shield duration also stacks if you do the tap twice when it resets, becoming around 8 seconds of shield. However, the shield duration can't go over 12/13 seconds, so holding down the skill twice will only stay at around 12/13 seconds.

There are several good 2 piece artifact sets that you can use for her: The Exile and/or Scholar, or Maiden Beloved. Use the ones that will fit your play style. Using a 4 piece set of any of them is also not a bad choice. The main recommended thing is to go for HP% main stat for your artifact pieces for increased shield strength, and a Healing Bonus% on the crown piece for her elemental burst. Energy Recharge% on the hourglass piece can be an option as well.

If you want to go for damage dealer instead, since her skills have a lengthy cooldown/cost, normal attack spam might be the better option. Go with any 5* bow for higher attack, or Rust. Compound Bow for free option. For artifacts, go with 2 piece Bloodstained Chivalry with any other 2 piece +18% atk, or go with 4 piece Retracing Bolide. The important one being Physical DMG Bonus % on the cup.

She works quite well when paired with most main damage dealers because of her Cryo elemental reactions, and her skills can be fired off pretty fast, while also not requiring her to stay on the field for continuous Cryo from her burst.

Diluc can cause 3x Melt reaction when used together with Diona's elemental burst for a significant damage increase. Razor's electro element is good with Cryo since it reduces enemy's physical resistance. Keqing is similar to Razor, but it will mostly help her if you go with mostly physical attacks.

Chongyun has trouble generating his own energy, so you can put Diona on the team as a sort of Cryo Energy generator in addition to her support kit. Having two Cryo characters will also give the Cryo Elemental Resonance for additional Crit Rate.

Normal Attack
Performs up to 5 consecutive shots with a bow.

Charged Attack
Performs a more precise Aimed Shot with increased DMG.
While aiming, biting frost will accumulate on the arrowhead. A fully charged frost arrow will deal Cryo DMG.

Plunging Attack
Fires off a shower of arrows in mid-air before falling and striking the ground, dealing AoE DMG upon impact.

Fires an Icy Paw that deals Cryo DMG to enemies and forms a shield on hit.
The shield's DMG Absorption scales based on Diona's Max HP, and its duration scales off the number of Icy Paws that hit their target.

Press
Rapidly fires off 2 Icy Paws.

Hold
Dashes back quickly before firing five Icy Paws.
The shield created by a Hold attack will gain a 75% DMG Absorption Bonus.

The shield has a 250& Cryo DMG Absorption Bonus, and will cause your active character to become affected by Cryo at the point of formation for a short duration.

Tosses out a special cold brew that deals AoE Cryo DMG and creates a Drunken Mist in an AoE.

Drunken Mist
・Deals continuous Cryo DMG to opponents within the AoE.
・Continuously regenerates the HP of characters within the AoE.