field > battle before > [physically, not chronologically] Menevia > (St David's; see HRB 8.16)
4 Which Uther with those foreign pagans held,
those foreign pagans > [the Saxons]
5 I saw a Saxon virgin, who felled 6 Great Ulfin thrice upon the bloody plain,
Ulfin > (Ulfin de Ricaradoch, who advised Uther to consult Merlin in finding a way to lie with Igerna: see HRB 8.19)
7 And had not Carados her hand withheld 8 From rash revenge, she had him surely slain,
rash > hasty had > [would have]
9 Yet Carados himself from her escaped with pain."
pain > troubles, difficulty
303.56