4 He smote thereat with all his might and main,
main > force, strength
5 That naught so wondrous puissance might sustain;
so > such puissance > power, strength sustain > endure
6 Upon the joint the lucky steel did light,
light > strike; fall, alight
7 And made such way, that hewed it quite in twain;
that > [that it] twain > two
8 The paw yet missed not his minished might,
his > [the paw's or the dragon's] minished > reduced, diminished