Monstrous Onslaught

• WWizards of the Coast2020-11-10
  Official Ruling
  You divide the damage among the target creatures as you cast Monstrous Onslaught. Each target must be assigned at least 1 damage.
• WWizards of the Coast2020-11-10
  Official Ruling
  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.
• WWizards of the Coast2020-11-10
  Official Ruling
  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.
• WWizards of the Coast2020-11-10
  Official Ruling
  The damage is dealt by Monstrous Onslaught, not by the creature with the greatest power among creatures you control.

Rulings via Scryfall