There is a stupidly easy fix for this. And that fix is that enemies should avoid walking off platforms if the remaining distance is enough to harm them, or place them in a kill area. Instead, they should take the quickest way out of the damage area that avoids this. It may involve adding some type of borders to the geometry that are only visible to the Boss. Atheon is stupidly rugged. Even with damage buffs he has an easy million health at minimum. He should not be worried about an aoe grenade at all but the tiniest health sliver. Even if the user is praising the sun nonstop.