As it happens, I've ended up with 3 Water SSR's: Nike Unleashed, Aphrodite, and Cthulhu. I'm working on leveling up the elite souls so that I can get Mordred and then leveling her up.

Aside from my SSR's, I have Undine, Rusalka, Circe, and Dagon all at lvl 50 or close enough to finish by the time Mordred is level 20.

Can anybody offer me advice on team combinations and who/how to best target SR's or SSR's to round out my team?