In your translation, I think that spjóti could be the instrumental dative
rather than accusative. If it is, I will be doing the happy dance because I
was able to recognize it. If it is, it would be translated " thrust through
with a spear."
Alan's translation below:
helst hlífa. Gunnar hjó undan honum báðar fætur. Kolskeggur lagði Karl
best protect himself. Gunnar hewed both legs from under him (ie he cut
off both legs). Kolskegg thrust
spjóti í gegnum. Eftir það tóku þeir herfang mikið.
a spear in through Karl. After that they took great battle-loot (booty).