I think the problem with Dark Ultimate is that it demands an entirely different approach for Light teams. For example, most players would advise going with Michael, Lugh, Shingen, and try and burst as often as possible. If a team with such a setup can't kill the Disaster fast enough, the DOT will slaughter such strategy.
Slashley said he only managed to AAB it after 2 years of play, with Andromeda, Sol and lots of Ascension. In other words, even a veteran would need to build a Light team with another approach if intending to kill Disasters reliably.
The Dark Ultimate is strong against a lot of common tools: Atk Down and Intercepts mean nothing, as the thing will use status effects more often than using a physical attack; I don't care about her Overdrives too much, so Black Propaganda doesn't seem that necessary; she applies 2 buffs to herself, so the Chaos Magic isn't as impactful. All her attacks are AoE, so Cover does nothing.
In the end, a team of five healers indeed seems to be the approach for dealing with that creature.