As long as Volbeat is in play, any damage done to Illumise by attacks from Fire Pokémon and Dark Pokémon is reduced by 30. You can't reduce more than 30 damage even if there is more than 1 Volbeat in play.
Flip a coin. If heads, the Defending Pokémon is now Confused. If tails, the Defending Pokémon is now Asleep.