In many such stories, the protagonist gains power gradually. At first, they may start with just their basic abilities. But as they face challenges related to the monster, they learn and adapt. They might learn new combat techniques, or develop mental strength. For instance, if the monster is a symbol of fear, the protagonist might gain power by facing their own fears head - on. This could involve having to make difficult choices that test their character. Also, the experience of past encounters with the monster or its minions can teach the protagonist valuable lessons, which in turn gives them the power to finally defeat the big bad monster.
Sometimes, the protagonist gains power from external sources. For example, they might find a magical weapon in the story world that has the ability to harm the monster. Or they could receive training or knowledge from an old wise character. Another way is through the support of friends or allies. Their combined efforts can give the protagonist the boost they need to defeat the monster. In addition, the protagonist's own willpower and determination can be a source of power. If they are resolute in their goal of defeating the monster, they are more likely to find a way to do so.
The protagonist can gain power through self - discovery. By understanding their own strengths and weaknesses, they can figure out how to best use their skills against the monster.