2020-11-10 : If some of the targets become illegal, the original division of damage still applies. The damage that was divided among the illegal targets simply isn't dealt; it isn't re-assigned to remaining legal targets.

2020-11-10 : The damage is dealt by Monstrous Onslaught, not by the creature with the greatest power among creatures you control.

2020-11-10 : The value of X is determined at the time you divide damage. It won't change later, even if the greatest power among creatures you control changes.

2020-11-10 : You divide the damage among the target creatures as you cast Monstrous Onslaught. Each target must be assigned at least 1 damage.