Heart-Piercer Bow (not the equipped creature) is the source of the triggered ability and the source of the damage. This means that a creature with protection from green may be targeted even if Heart-Piercer Bow is equipped to a green creature, but a creature with protection from artifacts may not.

If a creature is attacking a planeswalker, the controller of that planeswalker is the defending player. If a creature is attacking a battle, the protector of that battle is the defending player.