If your team lacks a healer or someone to perform cryo elemental reactions, Diona is the best out there.
She is one of the best support characters in the entire Genshin.
- Her charged attacks have cryo imbued that can help with elemental reactions with electro, hydro and pyro characters.
- Her elemental skill does some damage and more importantly generates a shield that can absorb damage.
- The elemental burst deals AoE cryo damage and also heals characters within the AoE.
She satisfies all the basic requirements of a support character – reactions, shield, and healing.
Now coming to the build, it depends on the remaining characters on your team.
If you don’t have any healers, go with the full maiden beloved artifact set.
If you need cryo and shield support, go with full noblesse oblige.
If you want all, you can go with 2-set noblesse oblige and 2-set maiden beloved which will boost the cryo AoE and also healing effectiveness.
You can concentrate on energy recharge stat as you need to fill in the elemental burst to activate healing. Healing bonus and elemental mastery are other stats you can prioritize.
For weapons, Favonious warbow is the best as the bonus stat is Energy recharge.