I'm using CoD, Lightning, Agrias, Tellah, Refia as my team now. I pretty much do the same thing every time and fights last 2-4 rounds.
Tellah casts Revive if there's a CoD, otherwise uses Boo!
Refia buffs with Embolden.
Agrias does Bladeblitz, Lightning does Area Blast, CoD does Barrage.
It doesn't really matter who Cecil guards, because usually it's only him and whoever he guarded left alive after the first turn. Once the dps has been thinned I take out Cecil with Full Break and Crushing Blow, and unless some serious healing happened the second Barrage usually kills whatever is left. I'm still kind of torn on Barrage vs. Sunbeam. Sunbeam is great the first couple turns since it's 999x2 to almost everyone, but it doesn't have the finishing power of an 999x8 Barrage.
I haven't fought any mega-whale teams yet though, since I'm pretty careful about who I pick fights with. Generally the lowest rank team that's got around a 1.2 multiplier. Just hit rank 7900 with a streak of like 133 or something. I've run into a few full-6* teams, like Lightning/Luneth/CoD/Refia/Cecil, but not the really insane ones that are nothing but Lightning and Luneth.