The shield that the monster created in front of him had already been burned by the fireball. But the monster immediately creates a second shield. On the other hand, he already separated the first shield from his body so that his body did not get burned.
But Lucas did not stop there as he continuously shot fireballs toward the monster and suddenly he used the energy booster to increase the power behind the fireball which immediately created a very big fireball compared to those other fireballs.
Those previous attacks already destroyed the surrounding area but the Monster could protect himself by covering his whole body with his branches to form a spherical barrier-like shape. But even with that, you can already see those branches got burned and The Monster already got hit by many fireballs.