Beowulf prepares for the dragon's attack by ordering armor made of iron instead of wood, because he knows that the dragon can breathe fire (on account of its destruction) and that wood will be useless against fire. (2335-2342)
During the battle, the majority of the dragon's attacks involve fire, but though Beowulf is well protected, his age is catching up with him, and he is unable to dispatch the beast; he shatters his sword in one swing. Wiglaf comes to his aid, though his hand is burned, showing that Beowulf's armor was indeed effective.
However, the dragon scores a bite in Beowulf's neck, which appears severe and bloody, but Beowulf is still able to kill the dragon with Wiglaf's help. However, soon after the battle, Beowulf realizes that the dragon's bite was venomous (2711-2715), and thus it was the unknown poison, and not the fire, which killed him.