4 Who, him rencountering fierce, as hawk in flight,
rencountering > colliding with; coming into hostile contact with
5 Perforce rebutted back. The weapon bright,
Perforce > By force
6 Taking advantage of his open jaw, 7 Ran through his mouth with so importune might
so importune > such grievous
8 That deep empierced his darksome hollow maw,
That > [That it] empierced > transfixed
9 And, back retired, his life-blood forth withal did draw.
withal > with that, hence: with the sword