Taking a deep breath, you charge forward and engage the Big One in combat. It's large and fast, but its movement is somewhat restricted by still remaining partially in the tunnel. For several tense minutes, you are the center of attention for everyone, man and lizard alike, all of whom watch the two of you spar.
Of course, by spar, you are mostly jumping around and trying to keep your spear up and not let it bite your head off. You keep trying to move around to its blind side, but the Big One is wise to your actions, and always manages to keep you in its line of site. You can feel your adrenaline starting to fail, and realize how stupid this is. It's not a question of if it will get the upper hand and kill you, but when.
A new sound enters your ears. It takes you a moment to identifying the high whistling followed by a soft thud, but as the creature roars you can plainly see the two spears that have lodged into its side. You risk a glance behind you and see a contingent of guards and the Princess charging up to the tunnel.
The guards fan out and act quickly. With the spears stuck into its side, the Big One can no longer back into the tunnel, as it is too wide. The guard prevent it from charging forward to get around behind you and let more of the small lizards through.
Before anyone else can act, the Princess has moved into the Big One's blind spot and charges forward, driving a spear into the creature's brain, killing it. As it drops to the ground, the other lizards lose their resolve and retreat back into the tunnels. A cheer goes up. The war is over!
For the next few days, you are a hero. The outsider who bravely pinned down the Big One long enough for the Princess to rally the troops and finish it off. A feast is held consisting of moss and, of course, lizard meat. They invite you to stay, but you long to get back to the sunlight, plus the food is pretty terrible. So, after a short while of basking in the glory, they guide you back to the surface, you say goodbye, and begin the trek back to the world you knew.
The End.