Answer:
The first disadvantage the hero Beowulf faces is the difficulty in locating Grendel's mother's lair. He tracks the blood from the mutilation of Heorot which leads him to a body of water, where he assumes that her lair is and dives inside.
Another disadvantage he faces is actually getting to the lair because according to the narration, he swims for a very long time underwater that is surrounded by monsters and dragons. When he everntually reaches her lair, he is ambushed by Grendel's mother and discovers that his weapons are useless against her, before eventually defeating her.