Answer: A supernatural plan
The battle between Beowulf and Grendel is one between the forces of good and those of evil. As Beowulf is our epic hero, his nature is particularly excellent. He is stronger, braver and more intelligent than most human beings. He represents the good. On the other hand, Grendel is notoriously bad. He is said to be "malignant by nature." We also know that he "has never shown remorse." The symbolic nature of the fight is meant to represent a supernatural plan, and to resemble the ultimate fight of the world, good vs. evil.
