To earn this bonus you will need to cause an extreme amount of damage to your target. You may earn this randomly, but it's a bonus that only comes with carefully lined up shots.
Your objective is to cause an extreme amount of damage to the castle to earn the bonus. You may find that this comes a lot easier in the later levels due to how elaborate some of the castles become, but the most straight forward way is to line up as many objects within your direct line of fire as you can. If you're having difficulty with this, you can use the bomb shot, line up a shot with plenty of objects to destroy, then allow your shot to travel through, then detonate your shot slightly later to cause even more damage.