"Secret Barrel" is one of the best cards for dealing Effect Damage. It inflicts 200 points of damage for every card in your opponent's hand and on his/her side of the field!
You can increase the amount of damage by using "Ojama Trio", which Special Summons 3 "Ojama Tokens" to your opponent's side of the field. This will be an additional 600 points of damage for "Secret Barrel".