This Mega fireball was the most powerful technique that he learned recently. He shoots the fireball toward the monster. Compared to his other fireball, your normal flame did not make this mega fireball, the blue flame made it. Yes, you guessed it right. His fire control technique reached the realm where he can use the blue flame. But as a side effect of this technique, he needed to use almost all of his spiritual energy.
Like Sam, others also attack the monsters with their most powerful technique. Mega fireball and those other spells hit the monster.
Sam didn't want to waste time so he quickly signaled the other warriors, who were waiting for his signal. Instantly, they attack the monster with their most powerful light spell.