David instantly uses the green shock technique that already exceeds the normal limit. He also uses the energy booster to break the limit.
The Boss monster who just appeared and already got injured because of those fireballs did not even get the time to react or prepare his attack. The moment he thought all the fireballs were complete, he got affected by the shockwave which instantly made him paralyzed and began to damage his body internally.
The Monster core of that Monster already begins to get affected by the shockwave. Every time the core gets damaged The Monster will feel unimaginable pain. Unfortunately, he is unable to move right now, and using that chance David once again attacks that Monster using his fireballs.
Those aren't your normal fireballs. He is already using the energy booster to increase the power of every fireball. As the monster was unable to move his body one by one he got hit by those fireballs.