Wherewith > With which
4 And double blows about him stoutly laid,
stoutly > boldly, bravely, resolutely
5 That glancing fire out of the iron played,
That > [So that]
6 As sparkles from the anvil use to fly,
use to > observe as a custom; hence: customarily, do
7 When heavy hammers on the wedge are swayed;
wedge > {Blacksmith's tool held in tongs and used on an anvil for cutting red-hot iron; also: ingot of gold or silver} swayed > swung
8 Therewith at last he forced him to untie