The Lamanites are trapped with no way to flee. They have an army in front of them, and one at their heels. They were fighting for their lives, and it is no surprise that they fought fiercely. The image used is that “they did fight like dragons.” Of course, there were no dragons in the Asian or European sense. In the New World, it might have been the image of a crocodile as the monster that inspired an image of dragons.
We learn a little more of the nature of Nephite protective armor when Mormon tells us that the Lamanites “did smite in two many of their head-plates.” Something other than a European helmet is being described, and it is likely that it was made of wood, and therefore being split in two.