It targets 1 monster on your opponent controls. Also, a monster that has been determined to be destroyed by battle cannot be targeted by this effect.[1]
The effect to decrease a monster's ATK/DEF by 500 applies as long as the target remains face-up on the field.[1]
When battle damage is inflicted, the effect of each "Roaring Earth" that is face-up on the field will be activated.[2]
Even if a face-up "Roaring Earth" is no longer on the field, the ATK/DEF decrease is still applied.[3]