Which eid should I use as my main for water team: Fernir 0* or the new MLB raid eidolon? My weapon grid for water is very weak and I use Arthur as my soul
Yam is Elemental atk, I think? So she'll do great once your Grid is good.
Until then, Fenrir will do okay. Jormund would be better, but it sounds like you're a new-ish player so I doubt you can afford to spend that many Eidolon Orbs for just one element.