able might > strength, capabilities
7 Its weary pounces all in vain does spend
pounces > claws, talons
8 To truss the prey too heavy for its flight;
truss > {Seize in the talons, usually in mid air, and carry off; used of birds of prey}
9 Which, coming down to ground, does free itself by fight.
111.20
He so disseized of his gryping grosse,
2 The knight his thrillant speare againe assayd
In his bras-plated body to embosse,
4 And three mens strength vnto the stroke he layd;
Wherewith the stiffe beame quaked, as affrayd,
6 And glauncing from his scaly necke, did glyde
Close vnder his left wing, then broad displayd.
8 The percing steele there wrought a wound full wyde,
That with the vncouth smart the Monster lowdly cryde.
1 He so disseized of his gripping gross,
so disseized > thus dispossessed gripping gross > heavy grasping; gripped load; strong hold; clumsy hold; or perhaps an amalgam of these ideas, as: heavy and strongly gripped load