4 And him with equal value countervailed:
value > valour; worth countervailed > vied with; resisted
5 Their mighty strokes their habergeons dismailed,
habergeon > {Sleeveless coat of chain-mail} dismailed > stripped of mail
6 And naked made each other's manly spalls;
spalls > shoulders
7 The mortal steel dispiteously entailed
mortal > lethal dispiteously > pitilessly entailed > incised
8 Deep in their flesh, quite through the iron walls, 9 That a large purple stream adown their jambeaux falls.
That > [So that] purple > red, blood-red adown > down jambeaux > leg-armour, greaves; or: leggings (perhaps of mail)