Yeah I heard Fischl is beastly but it takes a lot of time and resources to level up in this game. Especially when you're talking about getting decent artifacts on top of leveling the character itself, and upgrading the weapon.
I was bored today so I did a few domains for thunder based artifacts though....got some decent stuff just in case I decide to use Fischl at some point in the future.
I just wish I would've gotten literally any other 5* weapon besides bow.
Fred.
Good thing is I don't think you have to upgrade her weapon so much, just her level and her Oz skills. I never use her bow on enemies, just send Oz out and switch to someone else.