Superheavy Samurai Blowtorch

When an opponent's monster declares an attack while you have no Spell/Trap Cards in your Graveyard: You can send this card from your hand to the Graveyard, then target 1 "Superheavy Samurai" monster you control; change the attack target to it and perform damage calculation. Once per turn, when an opponent's monster declares an attack: You can change the attack target to this card and perform damage calculation. Cannot be destroyed by battle.