Boulderfall — ruling 1

You divide the damage as you cast the spell, not as it resolves. Each target must be assigned at least 1 damage.

Boulderfall — ruling 2

If some but not all of Boulderfall's targets become illegal, you can't change the division of damage. Damage that would have been dealt to illegal targets simply isn't dealt.

Boulderfall — ruling 3

If an effect creates a copy of Boulderfall, the division of damage and number of targets can't be changed. The effect that creates the copy may allow you to change targets, however.

Boulderfall : règles, fonctionnement et interactions (MTG)

Boulderfall is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Boulderfall deals 5 damage divided as you choose among any number of targets.

How it works : Boulderfall

Original English text — Wizards of the Coast / Scryfall

#1September 15, 2013· Wizards of the Coast
You divide the damage as you cast the spell, not as it resolves. Each target must be assigned at least 1 damage.
No other card is named in this ruling.
#2September 15, 2013· Wizards of the Coast
If some but not all of Boulderfall's targets become illegal, you can't change the division of damage. Damage that would have been dealt to illegal targets simply isn't dealt.
No other card is named in this ruling.
#3September 15, 2013· Wizards of the Coast
If an effect creates a copy of Boulderfall, the division of damage and number of targets can't be changed. The effect that creates the copy may allow you to change targets, however.
No other card is named in this ruling.