The instances showing Beowulf's superhuman strength are regularly expounded upon by the poet, who not only describes the feat, but mentions how no other single man could complete the task. The first of these that appears in the text is in the Beowulf's battle with Grendel. The other Geats leap to Beowulf's aid, but their weapons are useless against Grendel's tough hide and devious spells that "blunted every mortal man's blade." Be that as it may, Beowulf still managed to rip Grendel's arm off at the shoulder - and Beowulf used only his hands and beastly strength.
The second example I would mention is the giant-forged sword in Grendel's mother's lair. The poem describes this sword as "so massive that no ordinary man could lift its carved and decorated length." Not only does Beowulf manage to lift it from its hanging place on the wall, he also manages to swing it hard enough to cut off Grendel's mother's head with it.
Last, but certainly not least, is the presentation of Grendel's head to Hrothgar and the Danes. After killing the mother, Beowulf was still filled with rage. He went after the body of the already deceased Grendel and cut off its head with the giant-forged sword. It took four men to carry the head on spears back to Herot, but when they arrived, Beowulf single-handedly tossed the head down on the table in front of the Danes who had so recently deserted him.