I had a similar experience in arena a day or two ago. Mega whale team of 2 luneth, lightning, gilgamesh, and cod. I went first, raised CoD, then did some dps, but didn't kill anyone. On their turn they completely decimated me with all of them using aoe attacks... except my CoD lived. Apparently, every member of his team was using deathbringer, and CoD on diabolos was immune to dark damage, so she slowly barraged their whole team down over the next 3-4 turns while being spammed by immune's as the whales kept hitting him. I was amused.
Seems that a new strategy is getting popular where whales setup their arena team with no espers at all. That means, without espers adding fluff abilities into the list like bar-fira, protect, etc, units like Luneth, Lightning and such only have a few possible skills to use, so the AI almost always spams their aoe's every round.