Are you talking about the whole adventure, or about the three phases of the Battle (Clash, Threat, Finale)?
In the first case, there is no real question. If the heroes meet the big bad on the field and want to end it, then that’s it. It’s the final scene.
In the second case, it’s because each part of the Battle serves a different purpose. The players have to understand it. The Clash is the way you approach the Battle. The Threat is how the big bad acts (it or its minions). The Finale is… the finale.
So if you meet the serpent, you’re not directly, immediately fighting it in a melee. During the Clash you can choose your approach and try to get an advantage. It’s kinda the tactical phase of the battle. For exemple a PC could choose to sneak unseen while another provokes the serpent to distract it etc.