Change your:
Enemy|Spawn ExplosionDecal on layer("BG")
to
System|Create object ExplosionDecal on layer("BG") at (enemy.X + ViewportLeft(0) - ViewportLeft(1), enemy.Y + ViewportTop(0) - ViewportTop(1))
with the layer: BG
X-coordinates: enemy.X + ViewportLeft(0) - ViewportLeft(1)
Y-coordinates :enemy.Y + ViewportTop(0) - ViewportTop(1)
I think there a commands for that like CanvasToLayer or LayerToCanvas but I don't get it to work. Maybe someone with more experience can help with that commands.