What's the point in complaining about how others choose to bet a boss. Is it affecting your group? Is it going to change how your party beats Coil later on if someone else's group chooses to beat it this way? No. It's not. SE will sooner or later come out and says wither or not this is ok. If it's not, they'll fix it and ADS will just kill everyone when the timer ends. Until then, Do Turn 2 however your group see fit. Normal or easy mode. A win is a win. Later down the road if they can't beat later turns, not your problem.

Personally, waiting on a timer to end and fighting an enraged mob that was programmed by the devs to enrage that way doesn't sound to me like an exploit.