How could the following lines be paraphrased? taut throat, hell’s captive caught in the armsOf him who of all the men on earthWas the strongest.That mighty protector of menMeant to hold...
taut throat, hell’s captive caught in the arms
Of him who of all the men on earth
Was the strongest.
That mighty protector of men
Meant to hold the monster till its life
Leaped out, knowing the fiend was no use
This quote begins at line 788, in the midst of Beowulf's fight with Grendel. One of the difficulties in summarizing lines, rather than sentences or sections, of Beowulf or poetry of its lineage is the fact that many lines repeat or reflect upon information in the previous ones, so that they appear as sentence fragments or redundancies when viewed out of context (such as "taut throat").
"Hell's captive" refers to Grendel, in the sense that he is a damned creature and, considering the poem's strong Christian moralism, implying that he is disfavored and cursed.
The following lines simply describe and reiterate Beowulf's strength and role as a protector. Finally, they detail his intentions; to literally wrestle Grendel to death, or, more likely, to crush the life out of him.
So, these lines might be paraphrased as follows:
"Grendel, that hellish creature, was caught in Beowulf's grasp. Because of his great strength and protective instinct, Beowulf had no intention of letting Grendel go until he had crushed the monster to death."