A creature with 0 power can attack, as long as it doesn't also have defender.

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.

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.

If you attack with multiple creatures with power 2 or less, Raid Bombardment's ability triggers for each of them separately.

Raid Bombardment's ability won't do anything when a creature you control with power 2 or less attacks a battle.