Despite his shock, Finn managed to position and covered himself when he fell. Thus, protecting his vital parts and avoiding getting his shields broken. He already lost one shield prior. He only had two shields above him.
He didn't let himself stay for long on the ground. He rose and watched the scene before him in disbelief. The front part of his truck had caved in. There was no way that truck was still usable. Jack in his dragon form was standing in front of the wrecked truck.
While he was still processing the scene, he felt a sharp pain from behind. One of his two shields broke.
Grace had taken the chance and stabbed Finn in his back.
Finn only had one shield left. He couldn't afford to be careless. He took out a skill-free stone from his pocket. However, before he could use it, Jack had come before him.