What are the best artifacts to equip on her? Stats? Sub stats? Specifically
For Arena, typical nuker build, 100% crit rate, 170+ SPD and as much ATK and Crit Damage as you can get. Ideally, with CD gloves and in Savage set.
For Hydra, all of the above, plus around 250 ACC for Normal and Hard. If possible, give her a bit of survivability or pair her with good support, bc she is squishy.
Is her value in Hydra enough to use clan keys to get her? I have been using the keys for energy.
Do you mean Clan Gold?
You should be buying her fragments every week, yes. As far as I can remember you don't have a spirit nuker yet. She can do that for you.
The energy is pretty bad value, but if it's necessary, it's necessary. The fragments and accessories should be coming first, though.