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

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.

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.

The damage is dealt by Monstrous Onslaught, not by the creature with the greatest power among creatures you control.