> Helgi kom þá að og brá við skildinum og kom þar í lagið.
> Helgi came then at (him) and turned-sharply with the shield and (it)
came there into the thrust (ie intercepted the blow).

Although it's ambiguous from the morphology, in this instance,
parallel examples suggest that 'lagið' "the thrust" is nominative and
the subject of the clause: "the thrust landed in (it, the shield).

(1) og lagði Svartur atgeirinum til hans og kom lagið á herðarblaðið
og renndi út undir höndina

(2) og kom lagið í skjöldinn og renndi af skildinum í öxlina

(3) Lagið kom í skjöld Þorleifs.

(4) og kom lagið milli fóta honum

(5) Kemur lagið í fót honum fyrir neðan kné

> Hallkell hleypur þá upp á melbakkann og kemst eigi á upp annan veg
en hann skýtur (skjóta) niður knjánum.

> Hallkel leaps then up onto the lyme-covered-bank and (but) does not
make-his-way up onto (the top of the bank) otherwise than he shoots
(slides) down on (his) knees (does this mean theat he ended up on top
or at the bottom? To me it sounds like the bottom but HP&MM imply the
top).

'upp' is the direction he moves as a whole - up to the top of the bank
- but the only way he manages this is by thrusting downwards ('niður')
with his knees / throwing down his knees (onto the ground) and
crawling up it.