How does the poet foreshadow Grendels defeat in Section 11?

From the text:

The fact is, Unferth, if you were truly as keen or courageous as you claim to be Grendel would never have got away with such unchecked atrocity, attacks on your king, havoc in Heorot and horrors everywhere. But he knows he need never be in dread of your blade making a mizzle of his blood or of vengeance arriving ever from this quarter— from the Victory-Shieldings, the shoulderers of the spear. He knows he can trample down you Danes 600 to his heart's content, humiliate and murder without fear of reprisal. But he will find me different. I will show him how Geats shape to kill in the heat of battle.