Okay, so I went into the Maxwell fight with four 6* units and a FullBreaker and I still got my ass handed to me.
Current team:
Lv 55 6* WWFina (Cheer, Raise, some healing, Barelementra carrier, the weak link but still my second-best Healer, though I do have Garnet)
Lv 80 6* Cloud of Darkness (OmniVeil, Barrage, ManEater+?, no good weapons, but maybe I could make her into a mage? Or just toss all my ATK+ stuff on her and hope it's enough?)
Lv 80 6* Cecil (Focus, Curaja, I tossed my Barrage materia on him too. Great healbot and Cover tank. MVP probably? I can try to max him out?)
Lv 100 D.Lasswell (Nightmare strike, and uh... Blizzaga? Whatever, Nightmare strike and his insane ATK stat is probably good enough to justify him.)
Lv 80 5* Agrias (Fullbreak, Cleansing Strike, Cura, she's kinda frail and I can't seem to get any damage out of her...)
Potentials: Vaan, BC Lid, Mustadio, Garnet, Kefka, Tellah, Garland, Ingus, Celes, Terra, Golbez, Kuja, Zidane, Amarant, Charlotte, Rydia, Kain, plus nearly every 4* capped unit.
I'm thinking about trying to max out my other three 6*s and giving it another shot, just to see if the boost in stats is all I need, but I just don't know how I'm gonna cobble together the raw damage to beat her, let alone build up enough gauge to summon Diablos before she heals and debuffs me then multihit overkills everyone but Cecil in one turn...