Whenever a creature you control with power 2 or less attacks, Raid Bombardment deals 1 damage to the player or planeswalker that creature is attacking.
A creature with 0 power can attack, as long as it doesn’t also have defender.
If a creature is attacking a planeswalker, and that planeswalker leaves the battlefield, that attacking creature can still be blocked. If it’s unblocked, the attacking creature is still attacking. It simply deals no combat damage.
If the attacking creature leaves the battlefield before Raid Bombardment’s triggered ability resolves, Raid Bombardment deals 1 damage to the player or planeswalker that creature was attacking before it left the battlefield.
If you attack with multiple creatures with power 2 or less, Raid Bombardment’s ability triggers for each of them separately.
The power of the attacking creature is checked only when the ability triggers. Once it triggers, Raid Bombardment will deal 1 damage to the appropriate player or planeswalker even if the creature’s power changes before the ability resolves.