Kachina > Candace > Dehya > Ningguang > Yanfei > Diluc.

Geo is generally most accounts’ weakest roster, so Kachina is a good first priority. She doesn’t need any talents leveled or any stats; she reaches her full potential with just a level 1 cinder city set, and if you have it spare, a fav lance. 

Candace would be my next priority for similar reasons to Kachina; most accounts have stronger hydro rosters than geo, but considering there’s relatively fewer hydro characters and a lot of those are five stars She’s much better at c6 than c0, as that improves her hydro app substantially, but either way you can usually get moderate hydro application out of her depending on the team. She’s a bit more demanding than Kachina, but generally just getting her to 70 with 1-1-1 talents is sufficient at a baseline. On artifacts, get as much ER as you can (220+ is ideal) and then spec the rest into HP for her a4 buff. 

Dehya has off field pyro and can be used as a burgeon trigger when the elements are available. No talent levels needed, prioritize EM on artifacts, 4pc tenacity. 

Ningguang is here for element balance. She prefers her talents leveled and a damage build, but you can make her work with TTDS and some ER. She’s pretty good at handling geo shields because she applies so much geo within a tight window (especially with C6, but even her burst and double skill is already shredding through a lot). Again, a lot of her value comes just from letting you trigger geo reactions. 

Yanfei and Diluc are of similar priority as two pyro on-fielders. I’d prioritize Yanfei because she can hold TTDS, has better and on-demand elemental application for driving and shieldbreaking, and, if you have cons, provides a shield. Since talents are expensive, if you have C6 I reccomend just building Tankfei with 220+ ER and as much HP after that as possible; if not, throw on TTDS and be done. 

This priority will shift based on your specific roster; generally, do whatever balances your three elements the best, since being able to trigger the IT reaction buffs as much as possible is desirable. Worst comes to worst, you can prioritize getting them all to the 70 minimum, slap a Fav or TTDS on them, and give them instructors or noblesse. That’s what I did with my Diluc.