Boulderfall deals 5 damage divided as you choose among any number of targets.
9/15/2013 The number of targets chosen for Boulderfall must be at least one and at most five. You divide the damage as you cast the spell, not as it resolves. Each target must be assigned at least 1 damage.
9/15/2013 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.
9/15/2013 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.