Well, the good news is, Phul Guns are effectively the same as Ophiel Lances. You can make a Grid out of those. But... that's a loooot to farm. And if you want to FLB those Guns, you're gonna need a Wind team... ... ... though I'm not sure is Phul+ even out on DMM yet. Can't be bothered to check. Either way, the grind never, ever ends!Awakened Brahma is generally extremely strong, but needs to be at high HP, but thanks to being very tanky it's not a problem assuming you're running a healer. Anshar is very powerful with Tiara set, and you will absolutely need her against Ophiel+. Nuwa, as you expected, is very strong, but her team-damage is painful without healing. Athena is great against Water, and outside rather meh.
Justitia and Ymir are okay, but nothing to write home about. So for your last slot use Justitia/Nuwa/Ymir depending on what you feel like. Or you could run that Epic Quest SR healer.
PS. For both Ophiel Lance and Phul Gun Grids, a Phantom weapon is recommendable. But those something you'll naturally come across during Accessory slot grind, most likely.