his nursling > [Arthur] knighthood swore > took the oaths of knighthood
4 Therewith to do his foes eternal smart.
Therewith > With which (catachr., or misprint for "Wherewith") do > cause, deal smart > suffering, pain; grief
5 The metal first he mixed with meadwort,
meadwort > (Meadow-sweet, Filipendula ulmaria, a herb ruled in astrology by Jupiter and used for various officinal purposes. It is also the plant "that Queen Elizabeth of famous memory did more desire than any sweet herb to strew her chambers"; quoted Genders, 1971)
6 That no enchantment from its dint might save;
That > [So that] its dint > [the sword's impact]
7 Then it in flames of Etna wrought apart,
Etna > (The volcano in Sicily, which was very active in the sixteenth century; its flames were associated with Vulcan, who forged Aeneas's arms)
8 And seven times dipped in the bitter wave