Technically you CAN kill her before she shields up. You just have to have a few things fall into place for that to happen.
You need:
- High burst damage (obviously)
- Avoid her attacks that stack her shield bar (probably the hardest part but most important).
- Guess correctly and quickly when she splits into 5 copies (biggest RNG factor). This will stun her and reduce her def.
- Kill the first single orb she puts down before it goes off(not the 1 shot kill orb, just the pulsing one). This will stun her and reduce def.
If you can't do all 4 of those things, you likely cannot beat her before she shields since the bar seems to increase sometimes whether or not she hits you. Also sometimes it seems like she skips one or more of those vulnerability attacks like the copies or the orb, so I'm not sure what the variable there is.
If you don't have a big splash damage character like Hu Tao/Eula/Raiden, this is a lot harder to pull off. I've only done it once with a friend. I don't think it's possible solo since you need multiple damage sources coming in fast during her stunned windows.
Anyways, out of all the repeatable bosses, my favorites are Ganyu vs. Dvalin because it's over in like 5 charged shots and Shogun. I love the high skill ceiling on Shogun and she doesn't really have ANY invulnerability phases, but does have greatly reduced damage for the shield part. My least favorites are the wolf and Signora because of the long phases that you can't attack them. Particularly I hate fighting the wolf as any melee character because he always jumps back before he attacks and constantly goes out of range. He seems like a boss built to annoy melee characters. I usually just Yoimiya him dead in a few rounds, but still annoying.