Monstrous Onslaught deals X damage divided as you choose among any number of target creatures, where X is the greatest power among creatures you control as you cast this spell.
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 wonâ€™t change later, even if the highest power among creatures you control changes.
The damage is dealt by Monstrous Onslaught, not by the creature with the greatest power among creatures you control.
If some (but not all) of the targets become illegal, the original division of damage still applies, but no damage is dealt to illegal targets. If all targets become illegal, Monstrous Onslaught wonâ€™t resolve.