Forgive me, I don’t really remember splashy bucket too well,
The link skill isn’t bad (maybe there’s better options), but with mirror stance, she looks like you’re trying for a p/e phase res unit. Meaning, you really should give her reposition (maybe swap) to make best use of that link skill and swift stance, if you don’t change them.
If she’s for p phase sniping, though, I’d double up on the player phase skills. Drop swift stance for something, I’d maybe even consider death blow.
And if dragon slaying, naga would be her best friend if you can draw for her. Gives adv vs dragon and comes with draw back (unfortunately you normally won’t use it before attacking, so it probably won’t go too well with such an aggressive link skill, but a buff is a buff).
As mentioned, though, an unmerged Eir (especially without her prf) is probably stuck in AR, there are other dates I think you could merge up with feathers to do the role much better.