As a support, Diona is typically used for three things: Shielding, Healing, and Cryo particle generation. Pairing Diona with Cryo damage dealers such as Kamisato Ayaka, Ganyu, and Eula allows you to funnel energy into them while providing them a shield. Weapons such as and greatly increase her potential as a battery.
Diona's Skill grants a Cryo shield which allows cleansing. Every time you swap characters with the shield up, it applies Cryo. This acts as a cleanse.
Regarding Weapon Choices:
Sacrificial Bow: Preferred over at higher refinements (R3+) when batterying another Cryo character.
Silvershower Heartstrings: One of the best choice for stronger shields, however the Energy Recharge and useful passives of other weapons will generally make it a suboptimal pick.
Recurve Bow: One of the best choice for stronger shields, however the Energy Recharge and useful passives of other weapons will generally make it a suboptimal pick.
Sequence of Solitude: One of the best choice for stronger shields, however the Energy Recharge and useful passives of other weapons will generally make it a suboptimal pick.
Regarding Artifact Choices:
Full Stack HP% is recommended if you want stronger shields but a weaker heal. You can use Healing Bonus Circlet if you want a nice balance between the two.
Sets
Noblesse Oblige (4): This set provides a teamwide ATK% buff while also increasing Burst damage. Diona does not have the best uptime on this set, however, she may be the best option to use this set.
Instructor (4): This set is a niche option for Diona. It can be considered in teams which value Elemental Mastery and where she can trigger a reaction while on-field. Keep in mind that being limited to a 4-star set means your stat potential will be lower.
Tenacity of the Millelith (4): This set is a niche option for Diona. It can be considered in teams including some variants of Ganyu Freeze which have shorter rotation lengths, meaning Diona's Burst will not be used every rotation, and where you can snapshot the ATK% buff onto Ganyu's Burst.
Substats
CRIT Rate: Only recommended if using a Favonius weapon.
Regarding Talent Choices:
Prioritise whichever your Diona is geared for (shielder or healer), if built for a balance between the two, level both equally.